Human nature being what it is, the only way to discourage the big is to ban the small, however unpalatable that might be.
But banning it will have no effect, so dialogue is really your only option.
Why would it have no effect to have a sign proclaiming that it is illegal to leave anything on a SAM? Of course it would, maybe not with everyone, but as it stands that is the law anyway, and most people abide by it.