Sucks. I can understand that they won't help you, because they have to assume you're one of the evil haxxors unless you can prove otherwise.
I think it'd be pretty tough proving the account is yours when you just paid with game cards.
Anyway, here are my suggestions:
* if you EVER used a credit card, try to give them at least this number
* ask if they can check the credentials of the person who originally opened the account. If they can, offer to fax in a copy of your passport / drivers license, etc. to prove you are the person who originally opened the account.
* maybe they can check your connection history regarding the IP.
IPs can be traced to certain countries. If some dude from another continent is now playing on your account it's pretty obvious there's something wrong.
But still all this may not work, as they could argue that you could have sold your account and are now trying to swindle it back. It's really up to their goodwill I fear. I guess it would have been much easier if you had used any sort of credit card
Good luck though!