I don't see why you would lose them as source of feeders, just try to educate them about morphs and in that case Pastels, who knows maybe they don't know maybe they were themselves told those were Pastels, and as nicely try also to give them some hints on husbandry and the need of those animals.

If you treat people with respect or in a professional manner (without making a scene) you should get the same in return and might see some positive results.