To mark a thread as resolved, edit you're initial thread post and change the title with the added drop down.
There is a timed limitation on this however (3 - 5 days, George? We always forget :P), so after that point it cannot be edited.
If thats the situation, you can either leave it as unresolved, or it seems most mods don't mind if you use the 'report this post' option on you're initial to request a resolve for it.
header('HTTP/1.1 420 Enhance Your Calm');