Go into your PM's control panel and click "Rules, folders, & settings" under Options on the left:

Under "Add new rule" click "Sender" from the drop down menu:

Next, choose "Is foe" from the drop down menu:

Now Finally you can choose to "delete message" as I have it set, or you can make another folder for them to go in, or have them marked, etc.

If done correctly:

*Now whenever you add someone as a foe, they're messages will be dealt with (Other than that, I don't see the whole point of a "foe"). If this post is in the wrong place, or deemed too incredibly obvious- just let me know and feel free to move it to the right place/delete it. I don't know if this will be useful to that many people, but it's easier to follow then trying to explain it through chat.
-Matt "mharm2k6"