If you can do this by user and not by IP, you could just insert an HTML module on that page and set it to be seen only by a specific user. Then, in the module just add a javascript to redirect to another page. This would be the simplest approach, but I can't see how to make it work for an IP address.
For one site, I created a module that, when placed on a page, checks the Request object for the IP address, and redirects if the IP matches.
If you want to block certain IP addresses from the entire site:
http://support.powerdnn.com/KB/a370...-site.aspx