Jealous Associate

Okay, I've been trying to get this girl to stop being my associate for the longest! But somehow she always finds her way to be in actuvities with me, and I am avoiding her and not talking to her. She's not getting the picture. She believes I am her " best friend" and she is mine. I've expressed multiple times that we are not best friends(nicely). I am trying to cut her off without being rude(that's not my style).What should I do?