Most of the sessions I go to are of the "free-for-all" nature. We don't have someone in the "chair" trying to control it, because then it ceases to be a session and becomes a series of small concerts or performances. Which means that the person who goes to play for the fun of playing can't join in (or can but will probably be frowned at).
On the other hand, one of the (free-for-all) sessions here is suffering from "over-enthusiastic-guitar-man" who, just when a good momentum of Irish tunes has built up, completely destroys the atmosphere by getting out "101 favourite tunes for buskers" and attempting to play something completely inappropriate. We've tried being subtle, we've tried being blunt. He just won't take the hint. How do you deal with these folks?