Just to follow up on this. 

They guy was using slightly different email address each time. He was never confirming the account but his names would show up on the newest member part of the statistics box. 

I looked for his IP in the debug section but his account were empty and had no info. 

I managed to find his IP address on a site called Stop Forum Spam and then blocked it in my server. I think this should fix the problem, and he shouldn't be able to get onto my site at all now. 

I will let you know if he returns.