(I assumed you meant online) For paper mailing lists, there really isn't anything you can do. I had an independent UPS-style store for 29 years. I had 500 mail boxes and their mail never stopped. Even when returned marked 'deceased'. What you can do, if they have a paid return envelope, is to send back all their stuff in that envelope. It won't get you off their list, but it sort of feels good to know they are paying for your time.