A few weeks ago I had two women stop at my home to sell their Christian religion. I just told them no thank you I'm not interested and I closed the door. Not rude but firm.
No explanation needed it's not their business. Additionally when someone stops at my home uninvited and unexpected they should be completely understanding if i don't want to speak with them. At least that's how i see it.
This is conceptually the same thing I would say if someone offered me alcohol to drink.