For good you say? In that case, go all out!
Delete all characters, cancel sub, uninstall game, delete all files, remove all Warcraft related bookmarks, get rid of any and all things Warcraft in your life so that nothing in your immediate vicinity will ever remind you of WoW and potentially make you wanna play again.
Then you can quit for good!
But if you've got enough self-control, then just cancel sub and uninstall. That's it really. Then you can always return someday if you realize that the whole "for good" thing was inaccurate.