My boyfriend's best friend is an asshole to me. How can I get him to stop?

He constantly nitpicks my appearance and in front of me told my boyfriend that he could do better. My boyfriend just rolled his eyes and told him to shut up but he is still doing it. Why? How can I stop it? And yes I already told him to.
My boyfriend's best friend is an asshole to me. How can I get him to stop?
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