That actually happens sometimes. Someone used my email when registering for an online store, and now I get mail receipts for everything he buys. The thing is there's no way I can contact him. I tried contacting the store, but they didn't do anything about it either, so I just created a mail filter and got on with life.
I guess the real lesson in my case is: always validate your user's email address by sending it a confirmation link (by t he way, I think people using giant regexps to validate mail are missing the point). I see the point in letting the user log in right away (as soon as an email address and password are provided), but I think there still should be a confirmation email and the system should not send any email to unconfirmed addresses.