How to block a nested application but leave the main page available ??
can this be done by using AppFW + IDP policy ???
Yes. it can be done:
You would still need to see the details of the applications and the nested applications so you can determine what you want to block.
edit security application-firewall
create rule-sets and rule to match dynamic application BlockYoutube-Apps<===rule-set
decide if using whitelist/blacklis
Apply the rule set within a security policy for e.g.
edit security policies from-zone trust to-zone untrust
rule-set block BlockYoutube-Apps;