Paul, if you go to your USER CP...there will be a list of threads you are subscribed to. You can "Unsubscribe" to the thread you mentioned..
I'm assuming that you don't want the e-mail notifications that there has been a reply to your thread.
If you just no longer want to read replies on the thread, then don't.
If there is some problem on the thread and you feel you would like it to be locked....well, as the original poster of that thread, it is my understanding that you may make that request to a Moderator.