It's a shame OP's situation was solved by the player rage quitting. Personally I would have gone with a particularly aggressive Mega-Armored Nob (Chaos Champion, Dark Eldar Sybarite, Eldar Exarch/Ranger, Necron Immortal, Pathfinder/Stealth Suit, Lictor) to take him out, all in game, don't fudge any rolls. Hell, even have a Hark style Commissar show up and "replace" the sergeant.