::)I've advertised cars in the autotrader in the past. Always state no canvassers in my listings but still they come. One said "sorry i missed that" LIAR. These people are parasites. You're selling a car, someone will buy it, what on earth does that have to do with anyone else? I let one of them go through the motions after telling them i wouldn't send them any money which they agreed to. The conversation ended thus. "So sir for the small fee of £80".!!!!
Laugh at them & tell them in the politest way to take their departure. 
I used to play the "stringing along" game, but it dawned on me one day that my time is worth more than theirs.
Nowadays, as soon as I realise what they are up to....I get as much info about them as possible over the phone, then tell them that if they call me again then I will discuss it further with them in person.
Funny how none of them ever call back after that 
Before the days of mobiles i used to get calls from double glazing companies. Back in my pre mortgage days i lived in a double glazed council house & grew tired of telling them so. Just for my childish amusement i'd express a real interest, ask if they could hang on a second while i turned the dinner down & go & do something else for an hour or two, blocking thier line. You could hear them shouting "Hello"!!!!
No telephone exchanges nowadays but it was fun at the time. 
Modern alternative, which i did at Uni was to get the Arnie board, on your PC and some speakers, which has all his greatest hits from all his movies.
You control it via a flash board, has numous different 'yes' and 'no's and such, i think i had a 15 minute conversation once

best line is from Kindergarten Cop - 'who is your daddy, and what does he do?' has me in bloody stitches every time.