to OP:
Would you please be so kind to elaborate a bit on your post? As far as I understood your post, someone in your guild insulted you or your guildmates. I don't know what are your reasons for not asking your guild leader to remove that person from your guild, or if that person insulted all of you - why your guild leader didn't act on it by himself. Maybe simple solution would be to just place that person in ignore list so he doesn't bother you any more, but if that person was rude (I am guessing that was the problem, unfortunately your post didn't state that clearly) and if you want some action on it, perhaps some screenshots would be appropriate?
Best of luck in your endeavors in Azeroth!