What web browser are you using? I will assume you are either using IE or Mozilla Firefox.
How to block websites in Internet Explorer
1. Start the Internet Explorer, go to Tools / Internet Options
2. Go to the tab Content.
3. At the bottom click on Enable to activate the Content Advisor.
4. Next, click on Settings and go the tab Approved Websites and add all sites that you want to block.
You can block all subdomains of a website by entering *. in front of the domain. So if you want to block all sites of google.com, simply enter *.google.com
5. Back on the General tab, check the option Users can see websites that have no ratings
6. Click on Create password to set a password for editing websites and to disable the Content Advisor again.