Other than not drop 3.5k cs of troops into a region which they just took over and which had a lord of unknown trustworthiness? Nothing.
Well they have to play the hand they were dealt as well. Dropping militia into a contested region seems good practice, especially if you are the kind of player that just assumes others will play "fair" I've seen similar bugs before, and generally the misplaced Lord stood down as a gesture of noble good will.
