Bandwidth is usually a scarce commodity which needs careful managing to maintain.  However there is always a demand on that finite resource, and never more than when there's a major streaming event such as the Olympics.  There are many ways to block or throttle bandwidth to specified sites, but most of these are expensive and almost certainly overkill for occasional usage.

For a simpler, cheaper solution why not investigate OpenDNS.  OpenDNS provides a host of management features including content filtering and specific domain blocking.  Customization is provided so if a page is blocked, the user receives a page that can include a custom message and your own company logo.

However the benefit of education cannot be overstated.  If you explain to users the potential bandwidth costs of various activities, they will generally alter their usage to accomodate.  Very few users intentionally perform actions that harm network performance.