Hook them up with good weed and tell them to pack it. I know I hate being a mooch and if someone packs me a couple bowls and I don't have any to return the favour I usually get them back later or even give them money or cook up some food to munch on.
If I were you I would tell them to screw off or just don't even offer them a toke just pass it to the guy whos cool and then take it back.