I don't know if a hotel could refuse to a provide room for a person who is a member of the KKK.  However, I do think a hotel could decide not to rent its convention space to a KKK convention out of fear of violence.  Because it is liable to happen.  I don't know legalities here, but if I owned a hotel and had convention space to rent, and the KKK wanted to rent that space, I would do everything in my power to keep them out because where they go, violence follows.  There are also so controversial that they may actually scare off other people from staying at the hotel.  

If I were required by law to do so and couldn't get out of it, I would rent them the space.

Somewhere and somehow we have to draw some lines around what is and is not acceptable.  We are terribly divided now.  It is interesting to me that what we are discussing is "discrimination" against white people and not discrimination against Muslims, Gays, Hispanics.  Interesting - can we discriminate among whites?