You need to prove that you own the account, by signing a message using an address used on your hacked account.
If he/she no longer has access to the account, then they can not sign the address linked to the account. It could only have been done when they were actively in control of the account. Which if they did he would have to follow the instruction in the link already provided on the first comment.
You sign the message using the bitcoin address affiliated with your account, not with the bitcointalk account itself. Or did I misunderstand what you're trying to say here?