Well, you can "unactivate" the phone by doing a restore through DFU mode, and starting again as if it was a new phone. That would solve this problem I guess.
How to do DFU mode:
http://www.youtube.com/watch?v=ofpinyGlQ8I&feature=related
(Mind you, if you're on a Macbook on 10.5.6 you should probably ignore this advice since DFU mode won't work)