The question says it all.
Simple:
When you see someone doing things like being innapropriate, bullying, excessive censor avoiding, and spam voting or having a duplicate account, you start an answer on the Ban Report.
Give a link to the offending user(s) profile(s) and list the things they have done to break the rules. Adding screenshots and witnesses helps immensely.
Post your answer on the Ban Report thread, im assuming you know where that is.
A mod can look at it and decide what happens to the user.