Personally, I am okay with real person fan fiction (obviously, I write it) while others are not. I think it's fun to write about your favorite band, actor, athlete, etc., but there has to be a point where the line is drawn. That leads me to...
I think, for me, the point where fan fiction would become "creepy" is when it's written about people you know personally like friends, teachers, friends of friends, etc. There is something about crossing that line with people you know that would just sit funny with me.
Now what about when you meet your favorite celebrity? I have seen a blog or two where people have said they met their favorite celebrity and decided to stop writing fan fiction about them after that. Maybe there's just something about knowing someone in "real life" that makes it a bit stranger to write fan fiction about them.
I think even if I met my favorite celebrity (Dylan O'Brien, cough, cough) I would still continue to write about him, unless he fell madly in love with me like I suspect he would. Then I would stop and just focus on our whirlwind love affair.