ABAKUS

It's very easy. If (or when) you decide you no longer want to advertise with AdWords, just login to your account and delete all of your campaigns. That'll do it.

One nuance to be aware of: AdWords bills you after you have received the clicks, rather than before. So when one deletes all campaigns there will usually be a follow up bill, covering the period from the previous bill to the date that everything was deleted.