OK! I'm coming out the other side of some kind of gastric virus/food poisoning/alien probing, so apologies for the delay in replying. I was barely able to move!
Anyhow, I think I've figured it out. You edit the first post (your own post) in the thread. Then you "Go Advanced". On that page, there are the mod options to open/close the thread. It just worked for me with a test account, so if someone else could just confirm the same, I can announce this reward as done!