Well, that was what rehab was about. It kept the player out for a day and if the mafia didn't trust him to stick with them, they could whack him that night before he re-enters the game. As well, like Vudu mentioned, he could be left in the dark. And since he's technically mafia, he should be listening to the orders given by the drug dealer of who to vote out and not making his own decisions.
Again, if he was refusing to cooperate, the mafia could whack him and try again. Maybe having the whammy choice was enough but I thought the mafia had enough protection for any recruitment problems.