What If Someone Receives a Cease and Desist Letter and He Does Not Comply?

Your next step would be to file lawsuits. Will it be expensive? Probably - you're talking about defendants around the country. Unless there is a provision of law (of which I'm not aware) which would allow all the lawsuits to be filed in your area, you'll have to file the cases in various courts. There are lawyers who specialize in trademark, copyright, intellectual property law - you'll almost certainly need one.