So how do I say no?
First, don’t use euphemisms—for example, “I don’t think so,” or “I’m not sure.” You want it clearly understood that you’ve said no, so speak with authority and confidence.

Should I give a reason for saying no?
In a word, no! Keep it brief and simple. You can say why you’re saying no, but don’t try to justify it or explain your reasons. Remember, you have a right to say no for any reason whatsoever.

Suppose the person refuses to accept no for an answer?
Calmly repeat yourself. Feel free to look annoyed, or simply shake your head. Any reasonable person should get it and stop asking at that point. But if he or she is unreasonable, be prepared to walk away.