Web Based Reputation Score is just that...a reputation.
We provide a higher score for more trusted sites and a lower score for sites demonstrating malicious behavior. Generally the neutral range of sites includes scores like -3.5, 0, and +4 so we are still scanning those sites for security threats and updating our databases frequently.
The fact that the site is assigned to a category touches upon the Cisco IronPort Web Usage Controls feature which is independent of the WBRS.
Be that as it may, you can create an access policy that blocks only against ART sites with a score of 0 to block using the settings in the URL Category and Web Based Reputation sliders for that Access Policy.
However, I would not recommend it because of the explanation above unless it is affecting critical applications.
Setting a policy to ALLOW should still report our data for that URL.