I just act as if they don't exist and walk by despite all the efforts they make to get my attention. There's something oddly satisfying about the displays of frustration it seems to cause in them. What's even more annoying though are the ones that go door to door.
the ones on the doorstep are the worst, because that leverages people reluctant to tell them to sod off. It's borderline abusive.
I tell them I'm not interested and simply close the door once it's apparent why they're there. Why give the chance to waste my time? Still annoying though...