Not too sure what could cause them to stay in the spool, you could post a snippet of the delivery log from one of these messages with emails and personal info removed. 

However, I'd suggest against bouncing spam as this could open you up to a backscatter attack where spammers intentionally send you spam impersonating someone and then your server will bounce to the impersonated users. This could cause other receivers to mark your domain as spam. Just a heads up!

Okay, I'll just delete them then. Thanks!