People who make guilds in MMOs generally do so for social and altruistic reasons. Running a guild is a lot of work, inevitably places you in a position where you will have to deal with confrontation but we do it in the hopes of achieving a wonderful social space from which to play our game.
Real life issues are generally a pain in the butt for guild leaders. We don't make guilds because we want some random internet person's political, religious or philosophical positions.
But just because we don't want it does not mean we don't have to deal with it.
If someone in your guild makes a character called Pleasenomore Neegroz boot them and report them to the game moderators. Then apologise to the rest of the guild for his/her behaviour and explain what action you've taken. (You may also wish to retain his/her IP address and RL details on file so s/he can't sneak back in on an alt).
If you ignore it first the black people will leave. Then everyone else who dislikes racism will leave. Then everyone who isn't an ignorant troll will leave. Then the ignorant trolls will leave because your guild is full of ignorant trolls and sucks.
The same goes for other types of discrimination including the ubiquitous use of faggots or gay as pejoratives. If you act early you'll have to boot one or two people. If you hide your head in the sand most of the guild will adapt to people using these terms and will self-select for ignorant dickheads. Good people will leave and Counterstrike kiddies will be attracted to your butthead-friendly atmosphere.
If you run a guild, make sure it's YOUR guild.
Oh and to Saylah, /hug.