I dont want my boyfriend to be friends with this girl thats interested in him?

My boyfriend and I have been dating for 6 months and I have known him for 2 years. He has a best friend with this one girl who he has known for six years. She is interested in him and he is well aware of that. She has flirted with him and a couple days ago she kissed him by surprise. He told me right after it happened. Now, I have asked him and yearn for him to stop associating with her. But, he is unwilling to give up his best friend even though he knows she hurt me and our relationship. She really overstepped her boundaries. Am I wrong for demanding that he give her up? What should I do if he refuses even more? We've gotten into out r worst fight yet over this and I want to know what the most rational thing to do is. Help!