Remove-MSMailboxes
Remove (delete) a list of Mailboxes. Use the -UnDelete switch to Undelete instead. When a Mailbox is Deleted it is moved to a Delete queue to be deleted at a later date. Deleting these Mailboxes again with the -AllowRealDelete will truly delete them from MS.
Syntax
Remove-MSMailboxes -MailIds (Int32[]) [-BatchSize (Int32)] [-AllowRealDelete] [-UnDelete]