You could use the staff posting modification and then use @Ausphotography's suggestion and just edit or add a last post to the thread, explaining why the thread is being locled. I assuming from reading the threads here that only your staff can lock a thread. Granted, an integrated system would be a bit nicer, but at least it would solve the immediate need and it would not have to be undone if a better solution came along.
Or have I just re-iterated what is already obvious.

In that case nevermind.