You asked a good question. Most guys, whether they admit it or not are sensitive...that's why they keep asking, they don't want to be turned down. Use the ABC's of turn down instead of I'm not interested...cause that's a sure way to get called a bitch. But why?, because a man is more complex than a general answer...I'm not interested to us means....she doesn't know what's interesting about me, thus she really doesn't know what she wants and she didn't say she's NOT single.
A = Affirmation
B = Be Brief
C = Confirmation
Example...."Hi my name is charles", "charles", "yea charles"
= (Affirmation.) You called him by his name, which means even if you're not interested, you were polite enough to acknowledge him.
"Charles, you really caught me at a bad time, without going into details...I'm not available to date or make new acquaintences right now"...
Charles's response..."when would be a good time" your response, " When I know, You'll know"
Charles's response..."How will I know without your number"...your response..."I'll find you, I promise".
= (be brief without being harsh)
"Thank you charles for taking the time to speak with me...enjoy your night"
= (Confirmation) Thanks but no thanks...nothing open ended, no promises....
Hope this helps.