I've only had to ask someone to leave once---I have had to tell someone to cut it out or leave a few times:
We used to host cast parties for a local community theatre---"that's so gay" was a fairly common insult among the younger attendees- I'd nicely pull them aside and let them know why it was not acceptable in my home and that I did not want to hear it again. Never had to ask someone to leave.
DH has a young coworker who can be fairly homophobic and racist---was here watching a football match and when one of the players, of Turkish descent, missed a goal he went on about "fish eyes" and a variety of other racial slurs. I told him if he wanted to keep using racist slurs while watching he needed to find somewhere else to watch the game (DH totally backs me up here). He shut up.
For a few years we were part of a group for a wine tasting evening that was hosted monthly. winners picked the grapes for the next event and losers hosted next. When we hosted someone new was brought along by others. Something in the news came up in conversations and he talked about "those n-words blah blah blah" We told him that type of language is not allowed in our home. He settled down for a bit, then started up again talking to someone else (so I guess he thought if he wasn't talking to us directly, he could be as racist as he wanted to be=---we told him to leave. Several guests thanked us as he was making them all uncomfortable--but most had kids in school with his and were afraid to say anything and make waves that could trickle down to affect the kids.
He was plenty old enough to know better. Never saw him again---don*t really care.
Um, the real world?
I find it hard to imagine that by teen years they've never heard it at a public event, large family gathering, etc. EVER.