Don't let it get to you. I joined a MP with a guy and was up around the 7th hole, he sent a message and asked if I wanted to do a Challenge match for credits...I agreed so we quit the game and he sent me an invite for a 500 credit game...I declined saying too much...let's just do a 100...well, we finally settled on 250 (I think it was) and after 3holes I was 3 up..he then started in with wanting to stop the match, and I said no...he then started in with how he was a biker, would find me, etc...I just laughed it off, won the next two holes and he split.
What I'm saying is there'll be a lot who will try to take you off your game by intimidation, just laugh it off, concentrate and kick thier butt!