To quit WoW forever:
1) Download hacks and get banned
2) Delete all characters
3) Set parental controls to 7 days a week
4) Cancel Battle.net
5) Uninstall WoW
6) Burn your game discs+game key
7) Enjoy Life
To take a nice long break:
1) Just uninstall WoW, its way to much of a headache to install it again unless you REALLY REALLY want to.