Is there a PowerShell Script to bulk unsubscribe members' inboxes from group?

Yes, simply use the Get-UnifiedGroupLinks to get the list of subscribers, then remove them via Remove-UnifiedGroupLinks. Quick example:

Get-UnifiedGroupLinks groupname -LinkType subscriber | % { Remove-UnifiedGroupLinks groupname -Links $_.PrimarySmtpAddress -LinkType subscriber }

Of course if you are running this against a big number of users, test it first or better yet, use a proper script instead of a one-liner.