hi
if you want to block youtube or any other site using SEPM,first you have to creat the fire wall policy in sepm to block the required sites ,and you have to impliment these policy on the computer for which you want to block
here is the procedure
Log in to SEPM consoloe,on the left side you can see Clients Tab, just click on that and in view clients ,select "My company" and Right Click on that and select Add Group and give the required Name like YOUTUBE and click ok,after this you can see the group YOUTUBE in View clients , just select This and in the righ side you can see Policies,select that and uncheck inherit policies and settings from parent group,
select fire wall policy under location specific policies , then Click RUles on the left side, then select add Blank Rule ,then you can see this in the firewall Rules List ,just change the name of this for identification, then double click on HOST ,then select Sourc or destination and write "youtube" or "www.youtube.com" in Remote host by adding and click ok.
now you have created the policies for YOUTUBE GROUP ,then select the required computer in Default group and move that to your You TUBE Group and update Symantec Antivirus in the clients.
it will block the site