When I do AAU, PAL, CYO etc...(non-school games), I try to avoid ejecting all the coaches so there is someone left and the team does not have to forfeit. If everyone has to be ejected, then do so, but I find it helpful to tell the coach that if everyone is ejected this game is forfeited. 99% of the time the coach will bite his lip. Saying something like "Coach, try to control yourself because if you are ejected and no one is left to coach, this game will be forfeited." If it is close, it usually always works...
Now, if it is a blowout -- ejecting him may be your best option
Joe