Ceck the thread

https://www-secure.symantec.com/connect/forums/block-particular-website

http://service1.symantec.com/SUPPORT/ent-security.nsf/2326c6a13572aeb788257363002b62aa/9c561a4628b3c9a44925747f007b19cd?OpenDocument

How to block/allow website access using the Symantec Endpoint Protection Manager custom Intrusion Prevention Signature policy

http://bit.ly/uLiS84

Video: Allow and Block websites using Symantec Endpoint Protection Firewall

https://www-secure.symantec.com/connect/videos/allow-and-block-websites-using-symantec-endpoint-protection-firewall

Article: How To Block Internet address via Sep Manager Firewall Rule

https://www-secure.symantec.com/connect/articles/how-block-internet-address-sep-manager-firewall-rule

Allow and Block websites using Symantec Endpoint Protection Firewall

https://www-secure.symantec.com/connect/videos/allow-and-block-websites-using-symantec-endpoint-protection-firewall

Blocking a Website using Symantec Endpoint Protection

Allow and Block websites using Symantec Endpoint Protection Firewall

http://www.symantec.com/connect/videos/allow-and-b...

Article: How firewall works

http://www.symantec.com/docs/HOWTO55054

Open Symantec Endpoint Protection Manager

1. Click on Policies button
2. Under view Policies > Select Firewall
3. Edit the existing Firewall Policy
4. Click Rules
5. Right Click Rule Number 2 and Select Add a Blank Rule
6. Right Click Under the Action and Set it to Block
7. Right Click on the Host Select Edit
8. Under Specify host names or addresses of computers that trigger the rule Select : Local /Remote
9. Under Remote Click Add Under Type Select DNS domain
10. Under DNS Domain type the name of the Website e.g. : *.symantec.com
11. Click OK and close the Host List Window
12. Click OK and close the Firewall Policy Window
13. Assign the policy to the desired group

Note : In the same way, if you add *.com in Step 11 it will block the entire range of .com websites
Note : Make sure that all the computers you are managing have Network Threat Protection installed or else the Symantec Endpoint Protection client will not have the ability to block access to websites

 It is important to have the Application and Device control and Firewall Installed on each Client machine.

Here are few Articles for the same:

There are few Threads on the same issue as well, I would suggest a quick look into the same.

https://www-secure.symantec.com/connect/forums/custom-ips-signature-website-blocking

https://www-secure.symantec.com/connect/forums/how-block-access-specific-websites-both-url-and-ip-address

https://www-secure.symantec.com/connect/forums/website-blocking-custom-ips-signatures

Check this thread

https://www-secure.symantec.com/connect/forums/blocking-websites