Mail that's marked as spam should generally not be piling up in any queues, unless you're saving the spam for manual review.
One of the ways to see the queue is via the command-line qmgr command.
If you're not saving the messages for review and are somehow building a backup of spam and particularly in an outbound queue, then you may have an open relay here, or a compromised client or compromised server credentials that are allowing the folks to use your mail server for spamming.
Per the OS X 10.7 man qmgr doc, the queue files are here:
/Library/Server/Mail/Data/spool/incoming, incoming queue
/Library/Server/Mail/Data/spool/active, active queue
/Library/Server/Mail/Data/spool/deferred, deferred queue
/Library/Server/Mail/Data/spool/bounce, non-delivery status
/Library/Server/Mail/Data/spool/defer, non-delivery status
/Library/Server/Mail/Data/spool/trace, delivery status