How can I close my thread for further replies?

I just marked a thread I had created in the Content Creation section (https://forums.digitalpoint.com/forums/content-creation.102/) as SOLD. There is an option – 'Auto-close thread 14 days after sale expires' which I forgot to set while creating the thread. Now when I enter the 'Edit Sale' settings to activate this option, it is not there.
I want to close the thread for further replies as the sale info it contains is no longer valid.
How do I do this now? Or should I…
How can I close my thread for further replies?

See original post by Content Maestro