If you go to the first post and modify it to add [Closed] or (Closed) or something like that to it to the subject line, then save, the thread title will change to reflect that. People will be able to tell at a glance what its status is.
When and if you decide to open it again, changing it from Closed to Open should do it. If it's been about a month or longer since your last post in the thread, posting a bump will send it back up to the active area.