I have a blog post with comments on. I want to stop all people posting any further comments but keep the ones that are currently there. How do I do that? Using Wordpress 3.5.1
You can change the setting in the Discussion post meta box:
(If you can't see this meta box on your post edit screen, go to the Screen Options tab and check the box next to 'Discussion')
Changing this setting will not delete existing comments. It will just turn off the ability to comment as long as the setting is disabled. It only applies to the post you change the setting for.