I agree with Jeri -- as fun as it is to think of comebacks, the truth is that many Christians hold as a tenant of their faith that everyone else is absolutely wrong. They aren't alone in this belief among religions, but there is a strong tradition of prostelitizing in the US, and any attempt to give them a come-uppance is likely to backfire when they decide you need to be "saved" even more. Quite frankly, they may not be able to consider behaving any other way if they feel it's their God-given duty to "spread the Word." They might, however, be ammenable to learning, and that would be a boon to all of us faced with prostelitizing Christians who act like used car salesmen with a lemon to get off the lot.

I don't know how preachy their introductions are getting. They might just be singing the songs they like and be totally unaware of what everyone else seems to consider in appropriate behavior. If the songs are inappropriate to the session (for example, modern Christian music on a folk session) they can be asked to pick songs which suit the session. If they Dug up some old spirituals, it might be more comfortable for everyone else.

Since religious music is a part of traditional music, I suggest the moderator tell them they can choose whichever songs they like, however they need to limit their introductions to music and not religion. Or maybe those of you who don't feel comfortable singing along with the choruses should simply not join in. It might get lonely up there... and they will certainly get the point. If they don't take the point and loosen up, the group can consider options from there.

But meanwhile, I think you give them the chance to be courteous -- especially if they are unaware that's what they are doing?