Some suggestions.

If they call when you have some free time you could engage in a debate with them and thereby stop them annoying somebody else who doesn't have the time; call it your good deed for the day!

My late mother usually managed to be weeding in the garden when they came. She agreed to listen to them as long as they helped her with the weeding; that way they did their good deed for the day.

Going back to first pricipals can be usefull when debating with them. Why do you need a creator? Why should a god be synonymous with creator, etc.

Have a pre-printed leaflet on behalf of the Humanists to hand.