As above, I would strongly recommend getting professional advice on this.
For the forum your account was hijacked, there are a few other things you could try to get your old account back. First, try sending a PM to an administrator and be sure to mention as much detail as possible to prove you were the owner of that account, e.g. your IP address, the e-mail address you originally used with that account, your date of birth (if you had in your profile), etc. Otherwise, they may wonder if you were really the owner of that account. By sending a PM, this way if your e-mail was getting stuck in a spam filter, it will definitely get through by PM. If you just want to delete the threads by your former account, use the “Report post” feature on each of them to attract attention to the moderators, explaining clearly why you want the posts removed.
As a lesson to anyone reading this thread here, this clearly highlights the consequence of using the same password on more than one website. However, it takes practice and can be done, e.g. my MyCE password does not have a word in common with any password I use elsewhere, the same with various other websites, remote connections and so on that I use. Of course I forget the odd ones now & again, but that’s what the “Forgot your password” option is there for and why it is very important to have a strong password on your e-mail account.