Same thing has happened to us. AdWords won´t tell us what we have done wrong. We really have asked several times, but same lame automatic answer. You are banned for life! We have given up getting to the bottom of this and have opened up several new accounts. They have been running for many years now. And they are not able to detect them. It was quiet easy to do this. All you have to do is to concentrate on what they log about you. Then avoid that. Delete some cookies and there you go.