Access Request Settings - Disable Allow access requests

You can use that kind of PS Script using CSOM 

<#
Source:
 - https://sharepoint.stackexchange.com/questions/219634/sp-online-powershell-web-requestaccessemail-is-not-returning-any-value
 - https://sharepoint.stackexchange.com/questions/241415/csom-property-for-access-request-group
 - https://msdn.microsoft.com/en-us/library/office/microsoft.sharepoint.client.web.aspx
 - https://techcommunity.microsoft.com/t5/SharePoint-Developer/Changing-the-quot-Allow-members-to-share-quot-SharePoint-site/td-p/18562
 

#>

[string]$MyRootWebURL = "https://tenant.sharepoint.com/sites/RootSite"
[string]$SiteOwnerEmailAdress = "youremail@youremail.com"
[boolean]$ChangeRequestAccessEmail = $false

[string]$username = "admch_fabrom@sgs.onmicrosoft.com"
[string]$PwdTXTPath = "C:\SECUREDPWD\ExportedPWD-$($username).txt"

$secureStringPwd = ConvertTo-SecureString -string (Get-Content $PwdTXTPath)
$creds = New-Object System.Management.Automation.PSCredential $username, $secureStringPwd

function Load-DLLandAssemblies
{
	[string]$defaultDLLPath = ""

	# Load assemblies to PowerShell session 

	$defaultDLLPath = "C:\Program Files\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.SharePoint.Client.dll"
	[System.Reflection.Assembly]::LoadFile($defaultDLLPath)

	$defaultDLLPath = "C:\Program Files\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.SharePoint.Client.Runtime.dll"
	[System.Reflection.Assembly]::LoadFile($defaultDLLPath)

	$defaultDLLPath = "C:\Program Files\SharePoint Online Management Shell\Microsoft.Online.SharePoint.PowerShell\Microsoft.Online.SharePoint.Client.Tenant.dll"
	[System.Reflection.Assembly]::LoadFile($defaultDLLPath)
}

Function Invoke-LoadMethod() {
param(
   [Microsoft.SharePoint.Client.ClientObject]$Object = $(throw "Please provide a Client Object"),
   [string]$PropertyName
) 
   $ctx = $Object.Context
   $load = [Microsoft.SharePoint.Client.ClientContext].GetMethod("Load") 
   $type = $Object.GetType()
   $clientLoad = $load.MakeGenericMethod($type) 


   $Parameter = [System.Linq.Expressions.Expression]::Parameter(($type), $type.Name)
   $Expression = [System.Linq.Expressions.Expression]::Lambda(
            [System.Linq.Expressions.Expression]::Convert(
                [System.Linq.Expressions.Expression]::PropertyOrField($Parameter,$PropertyName),
                [System.Object]
            ),
            $($Parameter)
   )
   $ExpressionArray = [System.Array]::CreateInstance($Expression.GetType(), 1)
   $ExpressionArray.SetValue($Expression, 0)
   $clientLoad.Invoke($ctx,@($Object,$ExpressionArray))
}

function Get-SPOSubWebs
{
	Param( 
        [Microsoft.SharePoint.Client.ClientContext]$Context, 
        [Microsoft.SharePoint.Client.Web]$RootWeb 
    ) 
	
	$Webs = $RootWeb.Webs
	$Context.Load($Webs)
	$Context.ExecuteQuery()
	ForEach ($sWeb in $Webs)
	{
		Write-host " -------------------------------------------------------- "
		Write-host "       -->> SubSite:", $sWeb.URL -ForegroundColor green
		Invoke-LoadMethod -Object $sWeb -PropertyName "HasUniqueRoleAssignments"
		$context.ExecuteQuery()
		Write-Host "       -->> Has Unique Permissions:", $sWeb.HasUniqueRoleAssignments

		if($sWeb.HasUniqueRoleAssignments)
		{

			Invoke-LoadMethod -Object $sWeb -PropertyName "RequestAccessEmail"
			Invoke-LoadMethod -Object $sWeb -PropertyName "MembersCanShare"
			Invoke-LoadMethod -Object $sWeb -PropertyName "AssociatedMemberGroup"
                        Invoke-LoadMethod -Object $sWeb -PropertyName "AssociatedOwnerGroup"
                        $context.ExecuteQuery()
                        Write-Host "            -->> Request Access Email Before change:", $sWeb.RequestAccessEmail, " - Member Can Share:", $sWeb.MembersCanShare, "- AssociatedMemberGroup.AllowMembersEditMembership: " $sWeb.AssociatedMemberGroup.AllowMembersEditMembership -ForegroundColor Red
                        Write-Host "            -->> AssociatedOwnerGroup Name:", $sWeb.AssociatedOwnerGroup.Title   -ForegroundColor Yellow			
			if(($ChangeRequestAccessEmail) -and ($sWeb.RequestAccessEmail -ne $SiteOwnerEmailAdress))
			{
				Write-Host "      ===->> Request Access Email to change"
				$sWeb.RequestAccessEmail = $SiteOwnerEmailAdress
				$sWeb.Update()
				$context.ExecuteQuery()
				Invoke-LoadMethod -Object $sWeb -PropertyName "RequestAccessEmail"
				$context.ExecuteQuery()
				Write-Host "   -->> Request Access Email After change:", $sWeb.RequestAccessEmail
			}

		}
		Get-SPOSubWebs -Context $Context -RootWeb $sWeb
	} 
}


cls
Write-Host " ---------------------------------------------- "
Load-DLLandAssemblies
Write-Host " ---------------------------------------------- "

	$Myctx = New-Object Microsoft.SharePoint.Client.ClientContext($MyRootWebURL)

	$Myctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($creds.UserName,$creds.Password)
	$Myctx.RequestTimeout = 1000000 # milliseconds
	$MyspoRootweb = $Myctx.Web
	$Myctx.Load($MyspoRootweb)
	$Myctx.ExecuteQuery()

Write-Host " "
Write-Host " ---------------------------------------------------------"
Write-Host "  >>>> # Server Version:" $Myctx.ServerVersion " # <<<<<<" -ForegroundColor Green 
Write-Host " ---------------------------------------------------------"
Write-Host " "

Write-host " -------------------------------------------------------- "
Write-host "   -->> RootSite:", $MyspoRootweb.URL -ForegroundColor green

Invoke-LoadMethod -Object $MyspoRootweb -PropertyName "RequestAccessEmail"
Invoke-LoadMethod -Object $MyspoRootweb -PropertyName "MembersCanShare"
Invoke-LoadMethod -Object $MyspoRootweb -PropertyName "AssociatedMemberGroup"
Invoke-LoadMethod -Object $MyspoRootweb -PropertyName "AssociatedOwnerGroup"
$Myctx.ExecuteQuery()

Write-Host "   -->> Request Access Email Before change:", $MyspoRootweb.RequestAccessEmail -ForegroundColor Red
Write-Host "       ==> Member Can Share:", $MyspoRootweb.MembersCanShare
Write-Host "       ==> AssociatedMemberGroup Name:", $MyspoRootweb.AssociatedMemberGroup.Title ,"- AssociatedMemberGroup.AllowMembersEditMembership: " $MyspoRootweb.AssociatedMemberGroup.AllowMembersEditMembership  -ForegroundColor Yellow
Write-Host "       ==> AssociatedOwnerGroup Name:", $MyspoRootweb.AssociatedOwnerGroup.Title   -ForegroundColor Yellow

if(($ChangeRequestAccessEmail) -and ($MyspoRootweb.RequestAccessEmail -ne $SiteOwnerEmailAdress))
{
	Write-Host "   ===->> Request Access Email to change"
	$MyspoRootweb.RequestAccessEmail = $SiteOwnerEmailAdress
	$MyspoRootweb.Update()
	$Myctx.ExecuteQuery()
	Invoke-LoadMethod -Object $MyspoRootweb -PropertyName "RequestAccessEmail"
	$Myctx.ExecuteQuery()
	Write-Host "   -->> Request Access Email After change:", $MyspoRootweb.RequestAccessEmail
}


Get-SPOSubWebs -Context $Myctx -RootWeb $MyspoRootweb

You just have to set the variable defined at the top.

Take care, the group option available since some weeks is not manageable via PS for the moment.

Fab