If you want to remove a permission from a user, you can do so in the user's profile by applying a negative permission. In the profile, click on Permissions, and then navigate to the permission you want to revoke (e.g. "View Attachments (Generally)" or "View Attachments (This Forum)"). Click the permission name to bring up a popup. Select the box for "Deny This Permission" and click Update. That will effectively revoke the permission from the user completely, regardless of whether that permission is granted via any power groups or premium groups that the user is a member of.
Let me know how it works out for you!