We have a message queue on our queue server to which no-one has access. During a test we removed all the users from the queue. Now no-one can access it at all.
There is a file in the system32\msmq\storage\lqs directory that contains the configuration for this queue. Find the file that contains the name of the queue you are talking about. If you delete the file and restart MSMQ, the queue has gone. Obviously don't delete any of the other files!