Most Helpful Guy
You don't have to be friends with someone you don't want to.
And you definitely don't have to talk to someone who doesn't share your feelings.
Just stop contacting her.
You don't owe her an explanation. It's pretty self-explanatory anyway.
And explaining that you can't be her friend seems excessive. She can't possible care thaaaaat much.
There's a whole world of potentially interested women for you to explore.