An account is really never 100% recall proof. To take the best measures look here:
[GUIDE] The Best Way to Keep Your Scammed Accounts!!! - MMOwned - World of Warcraft Exploits, Hacks, Bots and Guides
I would start by buying WoW, TBC, & WoTLK if you are very serious about keeping the account.
Then transfer the character to your account, then finally ban the old account using this method:
[How to] Ban An Account Quick - MMOwned - World of Warcraft Exploits, Hacks, Bots and Guides