id say, " aWw your so sweet" and turn away.
"I am very fond of you, too. You are a good person. But.....I am not in love with you. I am going to free you so you can find someone who is right for you, someone who can appreciate you in a way I can't. You deserve someone who can reciprocate your feelings. And, even though I like you, I have discovered that I am not that person. I know you will have a wonderful future, but not with me. Goodbye and good luck."
The mean thing to do would be to hem and haw and equivocate and lead them on and allow that person to gradually figure out that this passion is pretty much one sided. Why drag it out and torture him/her? Tell the truth and move on. You have a right. And you will free up this person to find real love.
Find as less hurtfull as possible way to let the person go
Telling the truth is not being mean. Don't drag it out because that is more painful. Nip it now!!
Honesty is not mean. Dishonesty is.