In a game like this I couldn't care less if a bunch of racists decided to group up together.. I say let them create their guild and keep there hate mostly contained..
The thing is, when you get that kind of racism and hatred with people it only gets worse when they are grouped together. It gives them validity and makes them more apt to become more of the same. However if they where to become part of more diverse guilds many would probably become a bit more tolerant toward others of different races. The same thing happens in the work place, when people of different races are forced into the same working conditions and work together every day the people in the group who are racist usually end up being quite a bit more "tolerant" and some even become friends and change their minds.