Hi Hannes1985.
If I'm not wrong you have the transparent proxy enabled and you need only the specific MAC listed in ebox can surf the web. For this, first you need set up the "Default policy" to "Always Deny" in the left menu "HTTP Proxy" / "General" this change block any request to HTTP Proxy and then you need create the exceptions for determinates objects that you want allow use of HTTP Proxy, this you can do it in the left menu "HTTP Proxy" / "Object's Policy", only need select the object and set the policy for him, for example "Always allow".
Well, I hope to have helped. Regards.
PS. Remember when you add a object's member as 192.168.1.18/32 refers to a host.