I dont see why with all the genetic agriculture we do we can no create a non opium poppy that would interbread with their crops and neutralize the effects of the opium. Seems like it would look like the same poppy but not have the narcotic effect and they would have to harvest them all before they knew which plant was viable and which wasnt.
One problem with that is that there is a lot of opium produced for legitimate purposes. A modified opium free poppy vigorous enough to pollinate and overtake illegal poppy fields would also destroy the legitimate poppy fields.