I need to know how to find an IP address?

I run our company blog - we have quite a decent following, but we are getting individuals that are spamming in the comments. We delete their accounts, but they create a new one and keep spamming. How can I find their IP address to block it from joining and commenting on our site?

We have a Focus Community member that is looking for some help with this - any insight?

http://www.focus.com/questions/information-technology/need-know-how...