This situation got *so* annoying when I played with a 1/2Orc Barbarrian that I started over and changed my primary to an Undead Hunter
Negative plane protection helps, but that random encounter had me for lunch since it seemed random, and wasn't able to prep for the bastiches.
Another thing to consider is *not* going to Mr Bayle right after you get the $15k, since it seemed like this was the trigger for them going after y'all. Let your characters develop into bad mo-fos and you may be eating them for lunch I did this and accumulated as much magic stuff as possible. Now with my Undead Hunter as Primary, I send her out to engage them and have all my other characters stand back with missile suport...
Happy Staking!