"Do you have a flat surface I could use to show you this?"
You are not crossing the threshold, especially not with a line as weak as that. Piss off.
The latest tactic seems to be simply ignoring you when you say "It's not a good time" or "I don't sign up for things on the door" and carrying on with the spiel as if you haven't said anything. Makes it easier to close the door on them, though, since it's not actually a conversation if they're not responding to anything you say.