"Deny
A denied permission always takes precedence. Denied permission at any level (user, group, or role) denies the permission on the object regardless of existing granted or revoked permissions for that user. For example, if user John, who as a member of the sales role is granted SELECT permissions on the customer table, is explicitly denied SELECT permissions on the customer table, he can no longer access it. Similarly, if the sales role is denied access to customer, but John is granted access, he is denied access.

Note Microsoft® SQL Server™ always processes denied permissions first. If you deny permissions to public, you prevent anyone from accessing an object, including the issuer of the DENY statement."

BOL - Administering SQL Server - Managing Security - Managing Permissions - Resolving Permission Conflicts