Need help with someone selling a counterfeit product of mine, amazon won’t do anything

Over the past six months I’ve been trying to battle Amazon because there’s another seller who is selling on the same listing as me. My company is brand register and we do not authorize any other people to sell our product. When I try to contact Amazon about this problem they keep denying my claims, I made a test but of this product and when I received it it was not my companies. It did not look anything like it(not the same label, no logo, literally a completely different product. When I send Amazon these pictures they keep denying that it is a counterfeit and won’t remove their listing, what am I able to do to get them from selling my listing/ what help channel should I go through, I have tried to escalate previous cases, contact brand registry & normal seller support. I am just a bit lost here and don’t know what to do besides start sending out C&D letters….