User Permisssions White List vs Black List
You would need two different user roles: one with the permission granted for most users, and one without the permission for User B.
In the world of security, grants or permissions should always be assigned according to the principle of least privilege (only what the user absolutely needs to do their job). As such, privileges should always be denied by default and assigned explicitly, as with a whitelist. They should never be assigned by default and blocked individually (blacklist). Implicit privileges, especially when combined with explicit ones, are more difficult to audit and troubleshoot. Keep it simple.