@Rameshwar, Can you expand on this a bit more. Have you monitored all traffic coming from a particular device ( alert on all URLs, enable interzone-default logging, all that fun stuff) and verified that you aren't accidently allowing this due to how YouTube actually functions when on a mobile device? You can effectively thing of YouTube on the browser and YouTube on a phone as two totally different things, go through the normal process of blocking a site and finding all required resources, forgetting everything that you already have down to block the web-based YouTube access.
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