I think the original poster was referring to a particular incident with a particular guitar player - probably not all guitarists in general.

I've attended sessions where one of the most intrusive instruments has been, on occasions, a piano accordion - and, believe me, when a piano accordion is intrusive, it really is intrusive!

There's very little you can do in the session itself. Words need to be said outside the session - and my view, for what it's worth, is that if something needs to be said, it should be the session organiser who says them. So - a word to the organiser outside the session might be in order. Take an opinion from him/her. It may be, for example, that the annoying instrumentalist is really only annoying you - or it may be that everyone else feels the same.

I run a session myself and, if there was someone who was as disruptive as you say, then I would undertake to sort it - outside the session. It hasn't happened yet, I have to say.