Repeated spam calls, no matter how often you ask them to stop calling.

This company probably calls me once a month, if not more often. I have repeatedly asked them to remove my contact details from their marketing database, but they always call back. I don't have debt, making me a very unlikely client, but they keep wasting my time and theirs, and they keep calling from new numbers to get around spam blockers. What do I have to do to stop the harrassment?