check this link, this should be helpful in achieving the desired thing

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

With Endpoint Proection, you can't block one URL. You can either block entire site or allow entire site with Firewall rule or Custom IPS signature. But you can restrict access for the virtual directory at the webserver that you intend to block.

for IIS follow the article below,

http://support.microsoft.com/kb/324066

Yeap, in firewall rule you've got a place to put Host so I suppose you can block all addresses from one host or none...

Do you mean you have published two websites in the same server and you want to block one among them?If yes go to Start--->Run ,type inetmgr and click ok.Here you can go to websites under the server name and you can find the websites published using IIS.You can right click on the website which is required to block and click on stop.Note that SEPM (AV server) will use one website (By default the port number is 8014 but port 80 also can be used) and if you stop the site non of the clients will be able to contact the server....

Check the below article which can be used to block specific website or URL.

Title: 'How to Restrict Users to Specific Web Sites by Creating Firewall Rules for Managed Clients'
Web URL: http://www.symantec.com/business/support/index?pag...

Here is a Video as well

Title: 'Allow and Block websites using Symantec Endpoint Protection Firewall'
Web URL: https://www-secure.symantec.com/connect/videos/all...