I think Niz said it best, they don't read the game names.
Ie. 'Experienced Players Only' and you get guys playing their second game ever joining.
Then to be the nice guy you have to almost follow the 12 step process:
1. Verify that they played in epoch II.
2. Tell them they need to play in new player matches.
3. Tell them why they cant play in this match because they always reply with 'i played single player'
4. Ask them politely to leave (the good people have already left on their own by now)
5. Kick them (they must be a dick or a kid at this point and beyond)
6. When they rejoin ask them politely to leave
7. Re-kick them.
8 - 11. Re-kick them
12. Reform lobby because it crashed.
Personally i think that if someone gets kicked from a game, then they shouldn't be able to rejoin that lobby.
j