If you are really interested in getting this guy off your back, draw the line in stone, not sand.

Most guys do understand when a woman replies with a firm "No", and they usually back off. It's when you keep moving the line that they start playing around. So be strong and say "No, Thankyou, NOT INTERESTED". You have to decide what is more important to you, a few measly extra bucks or your self respect and dignity.