What might a gal say or do in order to convince a suitor that she is not interested in dating him?

Ask his sister out.

"Piss off" usually works well. :)

Introduce him to her “special friend.”

"No thanks.  I've already got one a**hole in my pants."

Look the Romeo in the eye and say, ”Look, I’m really not interested in dating you.” 

Doing this in front of other people, particularly his friends would be more impactful. 

If the dude asks “Why?”, just say you don’t want to embarrass him any further, and go on your way. 

One suggestion. 

The way its usually done is by saying very little, if anything at all. Usually accompanied with facial expressions like this; 


That way, it makes them appear less attractive, and make the man/suitor think; "she's awful! What a disgusting person for being such an obnoxious @$$ to me - I have to get away from her now". It's quite an effective technique. And clever. 

"Get lost creep!" might be helpful.