Then put up a sign and call the gendarmes if they trespass.
Advocating for a LAW like this is just idiotic.
I assume you're not on the Do Not Call list? Afterall, those telemarketers are just capitalists trying to make a buck. They probably even have a genuine interest in lowering your credit card rate.
The way I see it, the people in my locality have decided we are on a town wide Do Not Call list. There are ways to get my business that do not involve disturbing me in my home, e.g. leaflet my door, send me a piece of mail, walk with your shovel down the middle of the street, just don't interrupt me by knocking on my door and expect me to do business with you.