Thornir Alekeg
Albatross!
If this is only a mild frustration, first off I would talk to the players about it. Let them know that you are not opposed to an evil party, but that they ahve been crossing a line that is making things less fun for you.
After that I would probably try and see if I can make the game interesting because the tables are turned. I would not let them off for being PCs (I think someone already mentioned this). Let them feel they are always on the run, watching over their shoulders.
If you are trying to pull them back some, do the characters have backgrounds with people they care about? What if family members are run from their homes or taken hostage, locked up or even killed because of their relation to such evil characters. Or have family members try and appeal to them and make the stop.
After that I would probably try and see if I can make the game interesting because the tables are turned. I would not let them off for being PCs (I think someone already mentioned this). Let them feel they are always on the run, watching over their shoulders.
If you are trying to pull them back some, do the characters have backgrounds with people they care about? What if family members are run from their homes or taken hostage, locked up or even killed because of their relation to such evil characters. Or have family members try and appeal to them and make the stop.