How to prevent door to door salesmen please?

i've tried it with a sticker at the same height as our eyes when standing in front of the door, but those salesmen can't read I think... The sticker has a cartoon of a salesman at the door with a red X on top of it, next to it in my language it's in big letters "NO, I do not wish to buy anything, I do not need my chimney to get checked, I do not wish to talk about faith, SO KEEP OFF OF MY DOORBELL PLEASE. Thank you." LOL And guess what!!! They're still ringing my doorbell!!! I'm like smiling and saying "Yeeees? Can you please read this sticker?.." <putting the door half close and pointing at the sticker> "..Thank you, enjoy your evening Sir!"

Maybe I need to print it ten times bigger...