Firstly, you've posted in an ASP.NET newsgroup, so I presume you're asking
about how to prevent a computer with a certain IP address from entering a
site which you are developing - you certainly can't prevent a computer from
navigating away from your site and going to www.yahoo.com, at least, not
through ASP.NET...

You could do something like this in Global.asax:

void Application_BeginRequest(object sender, EventArgs e)
{
   if (HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"] ==
10.0.0.17)
   {
       HttpContext.Current.Response.StatusCode = 404;
       HttpContext.Current.Response.SuppressContent = true;
       HttpContext.Current.ApplicationInstance.CompleteRequest();
       return;
   }
}

This might be OK for a closed intranet application, but would be no use at
all for the live Internet because it's so easy to spoof an IP address...