Typically a thread does not get closed unless a moderator determines it's run its course and is no need for anything additional to be added to prevent the discussion from getting out of hand such as if an argument is ongoing in the thread and nothing constructive is happening any more.
You could report the thread and ask for action on it, but locking is at moderator's discretion. If it's possible someone could come in to seek further help with a similar issue, they won't lock it. You could also edit the thread title and mark it [Resolved] but that still won't prevent further discussion in the thread.