How to tell a good friend you are not interested and not ruin the relationship?

It's a problem indeed. I suggest that you'll try to fix her up with a friend of yours, by that you can win twice- she will be off your back and your friend will thank you for the girl. 

Be open with her. Tell her that you value her friendship and that you would love to continue being good friends without being negative in your approach. For more tips on friendship you can peep into

Perhaps if you have a girl friend you could work that into the conversation (or a fake one ) for convience.  Then tell her shes a great friend.  Try to keep the conversation light and by the way (How do you know she is really interested?  Tell us all so we know as well)

I was in this situation.  I was the girl.  He acted aloof and heartless when I just pushed him away.  When I stopped answering his calls, texts,emails then he got all sweet again.  He realized that i was the best thing for him, but it was too late.  i would not play his game.  and he lost out.