Hi,
You can use the Web Filter service.
It is not include by default. Go to Services > Package Manager page. Search for Web filter and install it.
Once installed it will be listed on Services. Go to Web Filter, enable it and select the Mode.
Operation Modes are:
Whitelist - allow every site included in the list and block everything else.
Blacklist - block every site included in the list and allow everything else.
Sites can be added under Site Blocking Rules. Then click "Save & Apply"
You can find the Wiki article in these Link.
Regards
AU