They can add a sort of reputation system, like in BG2. When your reputation was low, your party was occasionally waylaid by high level goody-goodies. Something like that, except these enemies should leave no loot whatsoever. This will prevent their intentional farming, leaving the murder-hobo path to the actual crackheads.
Or Flaming Fist can simply subcontract some Giths to hunt you down from above...
