I don't really see why it matters whether a Strongman keeps his title or not after usage. if the Strongman uses his ability on a "wasted target", e.g. one not being protected or having the bulletproof role, then why do people even care if the Strongman was used or not? Having some uncertainty is necessary for Mafia sides in games. If the Strongman kills a target not in need of a Strongman performing the killing, again, all you will know is that "there was a Strongman in the game!" if you kill the Strongman, and like I said it's in the game's best interest to have some mystery. In the other scenario, where the Strongman kills a target in need of the Strongman's power, e.g. a doctor protected target or the bulletproof, then it's obvious the Strongman was used if the doctor has enough info about his sanity to make a case, or the bulletproof had come out of the closet.
As it stands now I think the Strongman has a good role description.