Sorry, I completely missed the gateway connection. Late Night post. So yes you are right; you don't have a choice on Gateways. I read that in ettercap's man just the other day. LOL.
Edit: Well wait. If you need it to be offensive, then I think just use ettercap. Ettercap doesn't detect if you're on a gateway, so you have to tell it not to forward. This is unoffensive. I'm not a pro at ettercap, but I think this is true. Does this sound right?