Sorry for two memes a day, but I'm fairly certain we've all experienced a situation where we've wanted to use this response.

Best one I have used four times and it worked is
"I will give you $100 to Fuck off"

@sam9669 I used that in line at Walmart. Told the dude "Here's $20. It's yours if you shut the fuck up and leave."

@sam9669 @jhh2450 Here I have to agree with US terrorism policy. Don't negotiate or worse reward them. Just scare them off with brutal retaliation.

What worked for me on a very annoying and extremely pushy sales dude:
(He had a name tag with only his first name. So I looked him up on LinkedIn while he was annoying me. Was lucky that his house was for sale and posted all over)
"Listen mister <full name> living on <address> your job is to annoy people into buying shit. My job is to be creative on with computers. If you don't leave me alone I will dedicate this whole weekend to make life annoying for you and your family. Who do you think is going to win?"

He actually looked at me wide eyed and scared shitless. Than walked hurriedly away. Still wonder why the fear was that extreme. My best guess is that a few of those bad movies helped me out here.