Policy authoring help

Hi,

   I have a use case where in I need to write a policy to allow all users to do HTTP Post and block a certain group of users. I added a rule in the Groups tab to include the sender group to block HTTP Posts, somehow it is not blocking access to members of the group.

   Basically I am looking to write a policy which by default allows all and blocks members of a specific group.

Thanks for all your help.