Of course these guys are scammers. No legitimate company works this way.
All that you need to say is that you are not interested in a new roof, and that you will call the police if they come back. Then say good day, and close the door.
Don't engage at all with scammers, they are experienced liars.
There is no polite way to handle a situation like this. As other posters have said, this is an obvious scam and could warrant an alert to the police.
When I was a child, there was a group of people known as the Henderson's. Every spring, they would infest the area with offers to repave driveways, repair roofs and do other important home
repairs for a low price. Of course, the work was always very shoddy if not outright dangerous.
Old-timers knew about these grifters. However, the county had a lot of new residents who were
first-time home owners. They were ripe for the picking.
Once the Henderson's were detected, the police and the local radio station were alerted. The radio
station would provide daily alerts in their news shows and the police would keep an eye out at the entrance of new housing developments.