BTW, interested parties can also lock (close) their classified ad thread during the creation of the ad instead of having to go back to edit the initial thread post as indicated in the above post.
While creating the classified ad thread, down in the section called
Additional Options where you also go in order to manage attachments to add photos, there is a line at the bottom of that area where you can click the checkbox to close the ad immediately when you post it.
Just a FYI...