@Jakob Rohde The User would still be present you directory and would answer as to why you get the error when trying to invite them once more. Blocking is not the same as deleting a user.
Get-AzureADUser -Top <maximum nr of users you want to view> | Where-Object {$_.UserType -eq 'Guest'} | Select-Object DisplayName, UserPrincipalName, AccountEnabled, mail, UserType | Format-Table would give you a view of what Guest Users you have in your directory and their status.
Reference: https://docs.microsoft.com/en-us/powershell/module/azuread/get-azureaduser?view=azureadps-2.0
To run the commands above you need to have the Azure AD Module for Powershell installed.
You could then select the accounts you whish to re-enable from that list and run the following:
Set-AzureADUser -SearchString "Username@domain.com" -AccountEnabled $true
Reference: https://docs.microsoft.com/en-us/powershell/module/azuread/set-azureaduser?view=azureadps-2.0
If that is not what you are looking for regarding "efficiency" I would recommend deleting the user entirely from your directory as step 1 instead of disabling the account. This would however require you to re-add the user to the groups, role assignments etc that the now-deleted user once had.
Hope some of this helps you!
//Viktor