If they're being that pushy, tell them something like "I am happy with the amount of fish in my tank and I don't want any more. Please do not tell my son he can have more. It makes it difficult to (insert reason to be at store here) when he is insisting on more fish". Say it in a polite tone, maybe let them know you're serious in a discreet way. They'll probably back off a little. If they don't, I would get serious with them. If they keep it up even after that, don't go to that store, at least not with your son. Some people can be that way. Try not to shop their as much if the first try doesn't work.