How to control my jealousy?

My 24 year old gf of 3 years has this certain male best friend. The guy is not good looking, he is a druggie and somewhat of a loser. I told her to distance herself from him because I don't want him around in the future. She did not do so, because she is at the mall with him lol. I don't think I am mad because I am jealous, more mad because she does not listen to me. I am bothered and I wish she could see that. I am not being controlling.