Kind of like how fortress deals with neutral ragers and/or Nexus. Except in the case of the evil rager you're evil, so you get more leeway.

Don't go out of your way to attack them. This is especially true if they've never attacked you before. So if you're walking down eastern and some fort paladin just happens to be on the road then you give him the opportunity to be a coward and neglect to attack you.

If he does attack you then (IMO) you're free to chase him down and kill him. Though, obviously, mage-killing and the killing of cabal enemies should take always take precedence over chasing down some annoying fort paladin.

If some fort guy repeatedly attacks you and/or insults your character then I say you're free to actively hunt him/her and not just confine yourself to responding to incoming attacks. He has cabal powers, you have cabal powers, so it's not like you're pressing an unfair advantage (which might be the case if you were hunting down an annoying <i>uncaballed</i> paladin). Again, though, never do this when there's a mage or other unambiguous cabal enemy available to hunt.