M.R.
Here are some things you can try. Not all of them are appropriate. If he has an addictive personality then, like with alcohol, you will need to remove any temptation to 'fall off the wagon' (as in, an alcoholic probably should never drink ever again).
Cancel your internet service.
Like David said, cancel the credit card that is tied to the account.
My husband plays Warcraft, but never in a way that is neglectful to fathering duties, household duties, or husband duties. Message me for a way to get his account banned if the issue is that extreme. When users play the game, they agree to a certain set of rules. If they break some of the rules their account can be banned. Yes, this is a last-resort technique if all else fails. Yes, I realize this is a very shifty way of doing it.