You could look at this two ways.
1. the guy is truly mis-informed, and honestly thought what he was saying was the truth. In this case he needs to be "informed" , politely, and should go on from there.
2. He's trying to get people to buy his gear, steer customers from AONW and competitors and is knowingly spreading lies and half truths. In this case, he needs to be told he's wrong, set straight, rudely, and if it continues, a cease and desist order should be filed.
You could easily show a court that by him telling say, 10 customers per day, that airsoft is banned, that possibly that took 10 customers away from AONW and the likes. And has tarnished the NW airsoft industry because they now sell banned illegal goods to the public.
It's a stretch, but the guy should be put in his place. There are several other industries that this happens to all the time. Take body shops for instance, one says that 98% of all other shops do crappy work and are in the pockets of ins. companies, then that same shop has 10 adds that you only hear once because a lawsuit stopped them.