You will not be able to get there IP address UNLESS they send you an email and can't you block that person on the web site or ignore them.
You will be able to get their IP address in email only if they are not using email client such as Yahoo, Gmail or Hotmail. The email has to be from their ISP(internet service provider)i.e. SBCglobal, ATT, Cox, RoadRunner, etc. The IP address seen in email headers is that of the Hotmail or Yahoo etc...
Also, possibly if you use a Private Messaging client like Yahoo Messenger, WindowsLive Messenger, AOL Messenger; by opening a window to chat privately, and in the command prompt window type: Netstat -an, you may capture the IP address of the person who you're chatting with.
But having said that, why would anyone be obsessessed to want to find out someone's IP address because a person is being rude or obnoxious? (Im laughing too hard here). Hello, it's the internet!
And having said that,,, The girl at the dry cleaners counter and the Starbucks waitress is rude and obnoxious everytime I see them! Get over it. Who cares...?