Follow the above advice in order...I think everyone's ideas are stellar!
1. wonder how well a call to the superintendent of the school system, in order to discuss the possibility of you needing to file slander charges against the coach and the school system, would go over…..

2. I would file a complaint with his boss/principal and make sure something is on record. If he won't shut his mouth maybe a letter from an attorney threatenting a lawsuit would help and wouldn't cost too much. And like Spy said if that doesn't work hit him with a lawsuit.

3. start a rumor that he is having sex with underage girls.. or better yet.. that he sexually assualted you when you went there, and you are just now remembering it...(that would just be payback and now he's do 9 fold after this...+ it's funny!

4. As a last resort...and we promise not to tell anyone...Find out which car he drives. During school hours, (better at his house in the middle of the night), remove one of his tail-light covers and unscrew a bulb and dispose of it. Pull the wires off the back of the housing and pull the slack (they all have plenty of wire) out until it can reach the gas tank.

Feed the wires (formerly connected to the tail lamp housing) into the gas tank after removing the gas cap. Replace the gas cap and also replace the tail light cover, he's not going to see a small wire on the back of the car going into his gas tank.

He fires up the car, let's it warm up. He then goes to put it in drive or reverse, pushes his foot onto the brake, and BOOOOOOOOOOOOOOOOOOOOOOOOOOOOM.

Never even got a chance to get moving.

~SC~
5. Forget the whole thing...But it may be hard since you're in a small town. Maybe your g/f can call the police,principal and superintendent and say he touched her or made comments about wanting to have sex with her...Regardless, what he did was fuq'd up. That's BS.