What should I do about a "misbehaving" user?

This type of question really is one that crops up every once in a while - any sysadmin will eventually come across user behaviour that is potentially damaging, malicious or illegal.

Unfortunately it's been closed and I feel this would be an excellent candidate for reopening.

It falls within the domain of the professional sysadmin to have the knowledge and skills to handle this properly. It most certainly deals with servers and networking equipment as we can be called upon to deal with the resulting situation which may potentially affect the infrastructure and the business.