A mate of mine who felt a bit cheated in the deal of his last car and generally hacked off by the treatment of the sales guy left their sticker in but added one of his own across the top which read "Don't ever buy a car from....."
He got phone calls about that one, but steadfastly refused to remove his sticker. In the end the sales director invited him to a meeting to discuss his beef with the company; he got a set of new car mats and a free service on condition he removed the offending text.