psn details are in the system.dreg iirc. it's probably recreated when you do a system restore, since it houses other options. I don't know what would happen if you were to just delete it. I think it even has your password and email address.
the account id is all over the system, including the id.dat on the root of the memory card, so removing all traces of it may be impossible. your other option is account switcher to switch to a new account, but I have no experience with it.