This is a skeleton, Powershell. Needs prerequisite setup added.
$Cred = Get-Credential Connect-MSolService -Credential $Cred Install-Module MSOnline Get-MsolUser -ReturnDeletedUsers # The list of users/mailboxes marked 'deleted', is visible, if there are any.
# Do the next step ONLY if you are certain. There is no going back after this. Get-MsolUser -ReturnDeletedUsers | Remove-MsolUser -RemoveFromRecycleBin -Force