Omg what the heck I never knew how to close one without posting and always wondered how people did it LOL. I'm terrible at figuring things out, thank you for this information. xD
Also when posting there should be a checkbox to lock it (this is from memory...), but remember that you can only close threads that are on trading boards and the likes. Elsewhere, you can report the thread and request it is locked but the final decision is up to... someone.