Go to the Routers Web-page and select the Advanced Tab, on the left side, about the 7th entry is called Website Filter.
Click on it and you are able to entry sites not allowed to visit. Make sure you have set the option above the list called "DENY computers access to ONLY these sites" and fill in the URL, or just the domain name (xyz.com) to filter out all. Save the Settings and Bob is your Uncle.