You quite right, this is where the trail ends im affraid. Tracking down to a telephone number/address is quite hard past this stage. As said tho, you could hack approach the hacking side of things with a simple portscan (altho firewalls can get in the way of this, bring back sub7 :P )
Mail bombing would seem to be the best way, or just lots of sign up to ****** Also try adding to MSN, start saying u heared he had an account for sale.. blah blah, you know the drill on that one.