Working in such a big company myself (Large store of petstores) who dont actually sell either of these products, but anyway.
Unless someones cat is seriously made ill or killed from these products AND there is un questionable proof it came from a product you bought from them, in this case hartz they wont stop selling it, espcially if they are making a lot of money on this.
I know these products are horrible and by no means am I sticking up for these companies, but from working for a large chain of stores I can tell you no matter how many letters you send in or petitions, so long as the ££££ is coming in they will still sell it.
The best thing to do is inform people, stand near these shops and hand out flyers, they cant stop you doing that so long as it is respectfully, if a products isnt making any money then it will get taken off the shelves since its a waste of space then.
Many a time I have suggested things about feed where I work, and I get ignored and I work for them. We sell bob martin flea stuff for small animals, bob martin is rubbish I told the area manager that and all I got back was it sells, and it does. I cant say to customers dont buy it either because I would be sacked, which wouldnt be so good since I have a mortgage and bills to pay, but what I CAN do it NOT mention it at all..
anyway after loads of babbling inform people, if something isnt making money it will be taken off the shelves, and you wont get into trouble with any companies, who with their money and moutains of solicitors will wipe the floor with us lot!