Check out the "unofficial FAQ" maintained by LoisC at www.redundantcartridge.com/dbman - create a user/password and log in. There you can do a search on "IP" and it'll bring up several archived discussions regarding the pro's and con's of trying to ban people based on IP addresses. It's really a great resource if you haven't already checked it out.
The short answer to your question is it's possible but it won't be consistant or accurate. Several reasons listed are: not all IP addresses are available, many users have dynamic IP's that change everytime they log in, etc.