Usually people believe me when I say "No, thank you."
Even those people pushing samples in Sam's Club or at the mall or someplace.
I do remember one time when a fruitcake didn't believe me when I was walking through the mall with my wife and he was pushing some lotion or other and asked us repeatedly as we approached his stand which we had to walk by to get where we were going, even after we had both said no. He stepped in front of us with this crap in his hand and started his sales pitch.
Fortunately, I can still use my "prison work site 'Count Time!'" announcement voice, and did so in no uncertain terms.
Everybody for several yards in either direction stopped what they were doing and it woke up mall security. (vendors are not supposed to impede traffic flow)
On our way back through from what we were going to do, he ignored us completely.