Just keep banning them. The trolls will eventually get tired of having to go through 7 proxies and give up.
Without giving them too many clues, there is apparently software that will dynamically change your apparent IP every few minutes and not just by small increments in the last 3 numbers so you can't just block small ranges.
I know that. But trolls will eventually get sick of having their posting sprees interrupted, or having to go through slow proxies every time they want to post. Also there are shadowbanning techniques to throttle or ban users without them realizing they've been banned. It would be a lot better than nothing.