Question: How do I create a Group Policy that will prevent my users from accessing specific web-sites? I am at the 2008r2 functional level.

Answer: 
There's no built in standard way to achieve this using Group Policy.
For a Computer policy, one possible method is to use a hosts file. You could create a custom hosts file that contains the names that you want to block, and deploy that using the Group Policy Preferences feature to copy files. (Computer Configuration > Preferences > Windows settings > Files). 

Keep in mind that this approach may not offer the granularity desired in all scenarios, and you cannot block a tld in a hosts file, only specific names. It may also not be appropriate if the hosts file is in use for other pre-existing names. A typical entry may be: 

```
127.0.0.1 localhost facebook.com www.facebook.com  

```

Some domains are difficult to block due to the large number of host names and different fqdn's required. 

For the file source, you could specify: 

```
\\domain.com\SysVol\domain.com\Policies\{GPO guid}\Machine\Preferences\Files 

```

And be sure to copy the modified hosts file to that location. 

File destination would be: %systemroot%\system32\drivers\etc\hosts 

The "Action" would be "Replace". 

Note that the normal group policy refresh applies to this GPO. Testing has conformed that the file will be re-copied at the normal interval (by default, 90 minutes for a computer gpo).