At our local session, the four or five bodhran players who come regularly have learned to take turns on tunes that are suited to the bodhran. Luckily we (as I play -- yes, play-- the bodhran myself) all were able to come to that agreement easily and without any rancor. For some reason, our session seems to attract reasonable and (mostly) respectful bodhran players.

However when one brought a djembe and was trying out some new rhythms, that person was told quickly and in no uncertain terms to lay off. The djembe is a thing of the past.

Sometimes telling someone directly is the only way to go. It's not always fun, but it has to be done. Politely, if at all possible.