In the scenario you gave, it seems as though the OP has suffered twice. Firstly, the (alleged) hack and second being him not being able to restore his account in the forum. If the forum admins cannot be sure, they will not allow him access to the account. As he will probably not be granted get access to his forum account, it is advisable for him to at least re-register in order to start building up his new account. Though it will some take time, he will eventually make it on to a signature campaign and can start looking forward rather than back at what he lost.
For now he is probably very busy dealing with the other issue related to the hack about whether his personal documents and programs were compromised. Going through events such as these is a nightmare I would not wish on anybody.
The best course of action as far as proving account ownership is concerned is for the member to at least stake a bitcoin address. That way, if the worst should happen and the forum account is compromised, it should be relatively easy to sign a message and have access to the account returned.
It's unfortunately too late to do that in this scenario because the breech already happened, and unless OP already has such a signed message, it's not going to help him to prove he is the original owner by signing a message now. He can prove it by signing a message from any address he posted in an unedited Bitcointalk post prior to the hack, though.
An additional problem is that the admins are probably trying to figure out if the account was hacked or if it was the user who posted those infected files. It's unlikely it was the real harizen, but it's still an offence that warrants a permanent ban for the person who did it. That's why this case isn't a simple incident of someone having his account stolen and losing access to it. Content that is against the forum rules was posted from that account.