I have a good mate who has been a repeat offender committing "session sins". (Well, not at sessions mostly. Hippie jams really, but developing more and more of a Celtic flavour over the years). He sings too loud, takes over the lead if you sing a song he knows, and plays aimless and annoying lead guitar on tunes or songs he doesn't know the chords too. And when he learns a song, he usually changes the melody to suit himself--dumbing it down generally--and then because he's an alpha male, others in our circle end up thinking that's how it goes, and I they all start doing it WRONG (as in, not the way I learned it). Also, he's a bodhran player (shudder) and a singer-songwriter (double shudder). I stopped going to most of his jam-parties years ago cause it all wore me out. The points of conflict have been a source of considerable friction with us over the years.
However, he's a great friend and over the years has become fairly decent on the bodhran, and writes good songs. So, what to do?
I started a band, and invited him to join! The quid pro quo is that I'm the leader, and the objective is to get paying gigs, which has worked fairly well. This allows me to put up my arms and yell "Stop!!!!" when sins occur, and ask him to please just sing on the chorus, or the tag line, not noodle on guitar, rein in the bodhran somewhat ("hold down the rhythm" is any drummer's job, not "show how many licks they can give a goatskin in three seconds") etc etc. This has given me great satisfaction in a Svengalian way and it happens in fairly good humour because we all understand that's the deal. In a social situation at his house, or a pub session, I don't pull rank the same way...but I can see the changes. Definitely past Musicianship 101 now.
He gets to sing his own songs his way and contributes greatly to the character of the band. Once or twice the rest of us practiced without him and though the sound was cleaner we found we missed that particular energy he brings. And in performance it is there too and is a big part of our sound, especially his "ARRRRRRRRing" pirate song.
Doesn't work for every situation, but maybe since the UK has passed so many laws regarding sessions, they could institute "session boot camp" for repeat offenders...