Website blocking options available in Symantec Endpoint Protection Cloud (SEPC).
How can I whitelist a URL in SEPC?
Whitelisting URL's using SEPC.
Blocking URLs with SEPC is not supported, however, Symantec has a range of different products that have that capability.
Symantec Endpoint Protection Cloud (SEPC) does not have a family protection or web filtering aspect to it, so you would either need to get a subscription to Symantec Web Filter or some other 3rd party solution that does website blocking.