Yeah as others have said, you have to be straight up indignant about embarrassing her whenever she's around you. When she brings you food, knock that shyt out her hands onto the ground and tell her to clean it up while laughing.
When she comes over wearing some sexy shyt to try to entice you, laugh in her face and tell her it's pathetic that she thinks dressing up that old worn out p*ssy has any appeal to you and the women you're smashing now are much better looking and don't have
[INSERT WHATEVER SHE IS INSECURE ABOUT HERE]. Then slam the door.
If you do eventually succumb and take her down one last time, drink a shyt load before you do it. Like 13 shots of some bottoms shelf shyt, then while stroking, vomit in her face and laugh (do this at her place, obviously).
Basically you have to make her feel like shyt and then laugh in her face about it.
It is what it is...