The thread starter can always lock a thread if he thinks it has served it's purpose. I locked my thread about today's merit give away, as I thought it hd the potential to become a spam fest.
https://bitcointalk.org/index.php?topic=3136930.0A couple of things that thread starters may not realise are - You can unlock the thread, and add to it, or edit your posts, you can then lock it again. The other thing is that it looks as if merits can still be awarded to posts in locked threads.