Do you have a firewall and other general internet security? If not then that's where you should start.
Also get anti-virus software, ad-aware and spybot just incase.
Yeah, you can try Armor2net Personal Firewall which will be helpful to you. One feature of this firewall is Banned Sites List. You can enter the IP address of that computer into its Banned Sites List. That computer will be blocked. http://www.armor2net.com