Well in order to say "no" there has to be a question to which you can answer with "no." Like "Would you like to dance?" or "May I buy you a drink?" or "Would you like to go out with me?" or "Would you like me to f*ck you?"
I've had guys who would sit at my table (they would ask me if they can, but they are always acquaintances of my acquaintances so I have to say 'yes' and they count on that!). I am already in a bad mood cuz I know they will hit on me and they are completely not my type. So questions about my life start. I answer them with a smile, but keep a safe distance by answering shortly and not asking them anything. NOW ISN'T IT OBVIOUS ALREADY THAT I AM NOT INTERESTED?!?!?
It obviously isn't... So they ask me if they may come see my studio and I tell them it's closed for public. Or they ask me about the clubs where I hang out or sing and I say I don't know and I don't go out. Then I act like I am not interested, I look at another side, drink my wine, talk to my girlfriend, etc. Then they get it and either walk away completely offended or start attacking me that I am hostile or narcissistic or in a bad mood. So after wasting an hour of my life on people I never even intended to meet, they give me a hard time, because I am not interested in them, although I was totally kind and polite to them.
Now at which point should I have said: "I figure that you're making advances at me, but I am not interested in you"? The point is that these guys perfectly understand that they can't have me, but they just want to try their luck and think they have nothing to lose. So I waste my time on people I am not even interested in talking to and it's my fault that I wasn't thrilled about spending time with them. And usually these guys are way beyond any criteria of attractiveness (and I don't mean just physical).
I've had nice conversationalists in my company and I liked talking to them. And some of them were really nice guys. But interestingly, those guys would always figure out on time that I don't share the same desire of getting to know them better.
I think every guy needs to approach a girl gradually and read her signs; not to put her in a position where she has to say "yes" because otherwise she would be a total *****. If you approach a girl and say "hi" she has no choice left but to say "hi" back. Then you ask her questions and she can either answer them politely and ignore you in the breaks or pretend that she likes your company (let's suppose that it's not true) and after a while when you ask her for her phone number she says "Sorry I can't give you my number" thinking to herself how you bothered her.
I would recommend that you approach a girl and ask her a few questions and say a few things about you. If she seems to be interested (looks you in the eye, smiles, laughs, her eyes sparkle, her body language says "you make me feel like a woman", she asks question and shows great interest in what you're saying...) then you can precede. If she looks around, answers your questions shortly, doesn't ask you anything then she probably isn't interested.
If you're not sure then ask her if she would like to join you at some table or let you buy her a drink and if her reactions is basically "you go ahead, I'll stay here" then forget her!
So guys say women are inconsiderate, but I often feel that guys are inconsiderate when they approach me, bug me, waste my time, and pretend to not see that I am not interested. Moreover, if a woman is talking to every guy who wants to talk to her, how will Mr. Right approach her? Will he stand in a line and yell "Hey, hurry up, there are more of us who want to talk to her? Are you done yet?"
Finally, how would you feel if every time you go out, girls that you find totally unattractive approach you and want to talk to you, dance with you, buy you drinks, not let you talk to other women (cuz they are around all the time) and finally get completely offended when you refuse to go out with them?