If you want to close your own thread then use 'Report Post' button from that thread and request there to close your thread with valid reason. DON.
Just reply to it with "Mods please close this thread" or something like that. One of the mods will get around to it in a day or so.
In your topic/post click - Fill the description and request the moderators to close the thread or delete it.
Just drop in a PM to any of the mods. Thats the safest method of closing thread. I am sure that that will do your desired job easily.
Bad advice. That supposes we read all posts. Such request can sit in thread and never be read. As others have said use the bad post report.