Usually, for my cousin that hates BYU, and razzes me a lot, I know more about his team than he does. That usually shuts him up. I'll ask about the score of the games they beat us. He doesn't know. I tell him when he understands more about his own team then we'll talk.
If they know a lot about their team, then I just agree with them. Don't fight. The fact is they have scoreboard which trumps anything you could say, except maybe the national championship, but even that they ask what we have done lately. I just day stuff like, "Man, you guys own us. You are better than we are," etc. They stop bugging me when I concede. No use fighting against people looking to pick fights. Agree with thine adversary quickly is a scripture that comes to mind. Often they will then start talking more rationally about the games, teams and the rivalry.