Mailwasher is used to delete messages from the server before you download them.
Now your problems might be down to timing. If Mailwasher hasn't checked and deleted the messages before Mailgate goes to grab them then you'll still get the spam etc...
The mail still goes straight to your POP account but Mailwasher is designed to to allow you to peek inside and see whats there and delete any unwanted stuff.
Also Mailwasher only checks at certain intervals as set by the user.
Reading what you say above i don't think Mailwasher is the tool for you. Its really designed to wash individual mail accounts rather than mail servers.