There are several ways to quit WoW:
- Delete all your characters
- Sell off your account
- Get bored of the game*
* To get bored of the game, what you need to do is: start a private server, start a new character, and make it so that
you cannot let anyone else join that server.
After any of the aformentionned choices is done, then:
- Uninstall WoW from your computer
- Terminate your account (does not apply if your account has been sold)
Or, if you have sold your account, then the new owner may probably have changed the password so you cannot access it.
Otherwise, tear the game CDs apart by any means necessary.