I don't see why it's so hard to prevent already, just log the last 20 or so chat messages or pms that a player sends, but only make it viewable by admins if the player on the receiving end reports them, so that it wouldn't be an invasion of privacy(by reporting a player it tends to mean I want it looked into, so it's not like admins will be able to spy on messages I sent to a friend unless he reports me for some reason)
IMO thread should be renamed to harassment instead of racism, since it is a broader category and racism can fall under it.
In terms of enforcment, I think you need a better way of documenting punishments and warnings of a player. For example, if some guy is throwing our racist or homophobic remarks at someone to try and offend them, and an admin warns them, they could put a note and date. For example:
Skilliard's Behavior Record
11 June 2013-Harassment-noted by <admin name>
"Throwing out Racist remarks to player 12042 yet again, banned for 3 days"
9 June 2013-Harassment-Noted by snk
"Verbally harassing player12042 with racist remarks, given warning.
This way admins will know if a player has been warned before by someone else, and can punish appropriately based on past actions rather than just their own experiences.