>"sorry, I'm not interested"

This is the way.

For me, it helps to remember that they're trying to earn a living, as we all need to, and since I'm not going to buy anything, the kindest thing I can do is save them from wasting any time talking to me. So I'm polite, but super direct. And regardless of how they respond, I close the door, gently but immediately.

Likewise, when people solicit me on the street, I'm polite, but direct, and I don't break my stride.