If it were just somebody at a party I would ignore and avoid him. I can deal just fine with a little annoyance over a short-term period.
But my most relevant experience to this question when my best friend dated an utter asshat for our entire senior year of high school, and that meant seeing him every day, unless I wanted to avoid my best friend too (which I didn’t), so ignoring and avoiding wasn’t going to cut it. I don’t think I’d recommend handling it the way I did unless it’s a very close friend, but my friend was well aware of my dislike for her boyfriend, and she, fortunately, was usually quite aware of his flaws too and often understood why I was annoyed when I was, so I got to express my dislike fairly openly. Sometimes I had to leave the lunch room and go eat somewhere else to cool off while he was being an asshole. I just had to walk a fine line to make sure I wasn’t stepping on their relationship. My attitude about him to my friend was one of, “I don’t see his merits, but I don’t know him as well as you do. If being with him makes you happy, then I support it, but I can’t pretend to like him, either,” and she was understanding. It was quite the balancing act sometimes but the fact that my best friend and I have pretty much unconditional love for each other made it less stressful; I was never worried about losing her. Again, this would be a much harder situation if the friend weren’t so close; I don’t know for sure what I would do.
Incidentally, when my friend finally broke up with him, he went berserk and threatened to kill her. She told me she doesn’t think she could ever date someone I don’t approve of again. :D