How do I control who can comment on a particular entry?

You can set the comment security level for each individual entry when you post it. You can make an entry where comments are not allowed, even if you have commenting turned on in general. When you're posting an entry, change the Allow comments? setting to Disabled. If you have commenting turned off in your journal, you can't set one entry to allow comments.

If you make a protected entry, only people you have granted access will be able to comment. Anonymous comments are not allowed on protected entries, so commenters have to identify themselves on these entries.