It was a pretty dick move for Jim not to back you up. If you never had a history of cheating, it's a lot of crap for him to just roll over on you to keep playing a game. Besides, because poon couldn't match your skills doesn't mean he was right. Yeah, it might be a lot of crap that a "grenade" could take out an APC, if this was indeed the screenshot he had... but anyone who played the game would clearly see that you were using C4 to destroy the vehicle, therefore clearing your good name.
I congratulate your maturity when you went to face your accuser. "Picard out" I don't think was the line that was appropriate. Short of knocking all the cards off the table, it still would have been good to lean in close to him and tell him "Beware my APC-killing grenades, little boy", then smacked him upside the head (would have been fine since he was over 18 and unlikely that you would have gotten arrested for assault). Either that or tell him "MCox sends his regards", then went outside to slash the tires on all the bikes in the parking lot. A little passive-aggressive, and might have backfired if his mommy came to pick him up, but at least revenge would have been achieved. At least you took the high road, you surveyed the enemy, and realized what a pathetic piece of work he was. Mr. Flask should have told you that you were clearly the superior ninja in this confrontation.