Sometimes I just walk on by. Sometimes ...
"Just walk with me a bit, I've got an appointment ..." Then if they do ...
"I wonder. What proportion of net income ought we to give to charity? What percentage do you give for instance?" ...
Then follow this up. (Sometimes I have to insist: "No - let me talk. Now ..." They're usually trained to also listen to their mark.) They mostly don't have a clue - only chuggers after all - "So, what's your monthly net income after tax? And how much do you donate every month?"
With a bit of on-the-spot (or walking-briskly-along) mental arithmetic, something like, "That seems more than generous. Really? That much?" Or, "But that's barely three percent! You said ten percent!" Or, more often, "I know, you're not well-off at all, but we're thinking of proportion not absolute amounts! Surely you could afford at least a couple of percent!"
And (mostly, though not always, they lie), "And how much of that goes to (the particular charity they're selling)? Again, sometimes needs follow-up ...
Usual result? Faintly embarrassed chugger leaves me to walk on alone, mumbling to himself as he returns to his post outside M&S or wherever.
[Incidentally, I never sign up with chuggers, but I do gift-aid a regular fixed proportion of my net income to charity. I wonder what percentage people think I/we ought to donate?]