I thought there was a way to lock your own threads but I just tried to check and didn't find it. You can always just PM a mod and ask them to close the thread for you.
It's in selected subforums. In "General" you can't but in Re-Tail or VTP you can. It's been enabled in the places where it's needed most so that mods don't have to close so many threads in certain areas, since it used to be that many reports (lock requests) would come from Re-Tail/VTP since trades were often quickly completed.