If something like that were to happen and someone stole your account, your threads are safe as long as other people posted in them. You can't delete a thread containing posts of other forum members. You can only delete your own posts, move, or lock the thread.

There are, of course, other means of contact including Skype or social media. But I intentionally created a scenario in which I wanted the forum's PM system to be the only thing I wished to use.