This is going to sound mean, but there is absolutely nothing you can do but ignore it or report it. My suggestion is to disable message notifications to your e-mail/phone if you haven't done that already, because it could get rough.
My friends and I have 3 of these set up (one with 20 people, one with eight, and one with thirteen.) and the one with 20, we call the Thread of No Return. It was started when I got bored and group-messaged a bunch of my friends to play Truth or Dare and everyone decided to start talking instead. At that time, there was a 1000 post limit on each thread.
We made it up to 35 threads before the "thread branch feature" became interlocked with the old thread, so making new ones didn't seem important anymore.
To give you a time frame, I started it in the summer before my sophomore year, and I graduated almost a year ago.
Also for the record, my ex is in one of those threads, and he has me blocked. He can still see and recieve every message I send, but neither my name or avatar pop up. It's one of the weird ways around the block feature.
((hugs)) There really is nothing you can do, and I'm sorry you got stuck in one of these.
(Though, these *are* considered "abuse of Facebook's Features" because they ask that you not use messages as the chat application. You *could* report if you feel like making some people very, very angry, because if Facebook responds, it completely disables their ability to use the messaging feature for a while...)