If the carrier won't help and you can't contact the previous owner, then you are probably out of luck as those are the only ways to do it. O2 might escalte to RIM, but you would most likely need proof of purchase and then they might not do it either.
No back door or work around. BBs are pretty secure, you know.
This is one of the big problems of buying a used BB. Next time read the FAQ article about it.