You go to Security>IP Filter, For settings use this: Operation: Ignored
Direction: LAN>Internet
For source address you can leave it blank and then all computers will be blocked access or enter the IP of the computer you only want it to be blocked to. For Destination you enter the IP address of the website you want blocked (use IP Tracer to get the IP) , for protocol you can choose what ports to block of just select all to block everything.