Ignore them. If they ask something like, “Don’t you want to save the world?” say “No, thank you,” and move along. We don’t always have to be polite.
If they’re good at their job, they will have a response to every reason you give them, as you’ve seen. It’s no use coming up with excuses, just go.
And know that you can never donate enough to a major charity to make up for the amount they’ll spend mailing you every month for the rest of your life!