I dont really know any method to "unmanage" patches.
The custom severity might be the only things, that comes close.
Patch distribution should never be "careless" ;), but i do understand your idea behind.
Normally you create a policy for a bulletin only once and then modify the targets. So if you uncheck the update within the policy, thats like not active managing it. The custom severity helps you in reporting your "real" vulnerabilities.