To block a website you will want to set a Terminate Alarm on that website. Set the condition on the URL or Titlebar and make sure the terminate box is checked. Then that website will be unavailable to all computers being monitored.
Alternatively you could use "titlebar contains facebook" which is probably more dependable because it would work with any possible browser. (ActivTrak only extracts Url's from IE, FireFox, and Chrome)
You could add an additional condition such as "user equals userxyz" if you wanted to enforce only for a particular user.
Create additional alarms for other websites.
See attached image.