you can check out this link, more specific to blocking out with configuration within the httpd.conf file
http://httpd.apache.org/docs/misc/security_tips.html
not sure on specific users, unless you create a site, and make it so it is password protected or whatever.. something along the lines like that.
if you wanted to block ip's or whatever.. that you could do with iptables/ipchains... which if your not familiar with you can get sources for that from
www.linuxdoc.org
