You RLJ you get catched :(

Hmmm, it's not in thread tools and I can't find it in post editing mode... :(

1. Edit on the first post in thread (you might be able to edit others I've always used #1)
   
2. Go advanced
   
3. At the bottom, just above Submit, is an option to "Close this thread after you submit your message"
   
4. Check this and then Submit
   
   

It might be that only posts in the Classifieds can be dealt with this way.