I'm 18 too, so dont take my advice too seriously, but if i might take a stab at this :
Playing a game for 40 hours a week is f!cking whack. First, try talking to him softly and see if that works. What does he like to do other than play games? Play basketball? Then take him to a basketball court and play with him or take him out to a game. He'll then realise how good it is to NOT play wow and do something else.
If he really likes playing only video games and has nothing else that he does, try cancelling broadband access. How does he pay for his wow subscription? If he works and pays it off, thats cool. Still, try to put enormous financial burden on him so that he can see what it means to pay for all this crab sh!t. For example, if he works, tell him to pay for Wow AND internet access AND for his computer (somone must have bought it).
If that doesnt work, try distracting him from WoW. Notice the time he's going on guild raids (or whatever they call those things where you sign in regularly and play). Right during that time, order a FRIGGIN HUGE pizza and rent some REALLY good movies and eat the pizza and watch the movie with your family (assuming your bro doesnt join you). Make the place really comfortable, that is, dim lights, pump up the volume, and make it a really good experience thats not to be missed. Your brother will be really tempted at this. Two choices for him : play a sucky ass video game, or watch a really good movie with tasty food. If i were him, i'd take the second option.
If that fails too, without his knowledge, swap his computer's graphics card with a crappy ass one. Make his gaming experience a really sucky one - without his knowledge that you're doing all this. Because if he knows, then he'll take you for his enemy. That'll seriously suck as godknowswhat he'll do next. So, do all the tricky work behind his back. I know this sounds like i'm an a**hole, but YOU are trying to help HIm for HIS own good. You're not gaining anything by his not playing wow. Make him understand that too.
If all else fails, just let him play his stupid game. There's nothing else you could do. You tried your best and it didnt work. If he flunks out of high school, let him. Hes going to go thru summer school, which will tear him apart. Sooner or later, he'll realise that this game has brought his life to a record-low.
Whatever you do, dont take extreme steps such as break the computer into pieces in front of him, or kick him out of the house, or send him to a stupid counselling camp or something.
If it helps you feel better, a friend of mine is a Wow addict as well. He plays WoW a lot, dont know how much hours per week though. He even cut school days to play WoW. Now he works in kroger and pays for his own subscription. He tried talking me into playing Wow, but i said no way in hell am i paying subscription money for a fcuking game.