Hadoob024 is right. When you reply to their mails you are just letting them know that there is someone checking that account...hence increased mailings because your email goes onto a "Confirmed alive" list. Maybe just ignoring the emails and filtering out mailings based upon the domain name so you don't have to see them might be more effective.

Also another good point someone made somewhere (can't remember exactly where) is that the reason companies send out junk mail is because it actually works sometimes. Therefore, if you receive a junk mail for something, don't buy the product, especially from the company that sent you the junk mail. I mean if everyone just never bought anything from companies that used junk mail advertising, then they would have to stop using these sales tactics. Saving 30 cents on an ink cartridge probably isn't worth the hassel of having your email filled to capacity with junk...