We must distinguish between negation and negative wording.
Both of these work very well:
[ ] Are you happy
[ ] Are you sad
But none of these:
[ ] Are you not rich
[ ] Are you not poor
Likewise, the context is important. Let the meaning of the check mark be consistent. Like this:
[ ] Install software
[ ] Add start menu item
[ ] Add desktop item
[ ] Accept license agreement
[ ] Assign to news letter
Instead of this:
[ ] Install software
[ ] No start menu item
[ ] Add desktop item
[ ] Disagree with license agreement
[ ] Assign to news letter
The general best practice when it comes to filtering, is to check mark the content that should be included:
Use these sources to generate the list:
[ ] Google result
[ ] Bing result
[ ] Yahoo result
[ ] Ask.com result
But depending on the context, you could use negative wording without reducing the understandability:
Exclude the following content:
[ ] Exclude porn
[ ] Exclude duplicates
[ ] Exclude foreign languages
[ ] Exclude bookmarked pages
I'm not quiet sure which criteria you need to include/exclude, but this might be a suggestion to the three states you describe in the question:
[ ] Include used items
[ ] Include unused items
Include unused items. This, I would do for two reasons: 1) Positive wording, see @jonshariat's answer, and 2)Include all itemssays nothing about which items will be left out (the unused ones). – jensgram May 17 '11 at 19:04