This is all very hand-wavy. First of all, hellbanning is not always the same thing everywhere, so no, there's no guarantee that a serious troll knows how hellbanning works. More importantly, I have actually had to deal with forum trolls as an under-resourced dev for a very popular and long-running forum. It's an arms race, plain and simple. Information asymetry is to the developer's advantage. Have I gotten to the point of implementing hellbanning? No... but I understand the justification for why it's done.