I am trying to stop emails containing a string of text in the subject from being accepted/delivered.

I know there is spamAssassin which can be set for different levels of filtering but I find that it is somewhat inneffective on filtering spams. I also know about Boxtrapper which can be used to filter email rather simply but it adds another headache of whitelisting in order to receive emails where a user would have to respond back to an authorizion message sent automatically from Boxtrapper.

Does anybody know a logically way to just filter out unwanted subject lines from getting accepted/deliverd in cpanel?
Thank you -kevin