I'm also new to Squid things..Try this, its blocking the streaming but if the users have any youtube downloaders then vdo is getting downloaded. But the streaming is not happening.
acl audio_video_request req_mime_type -i ^application/x-shockwave-flash$
acl audio_video_request req_mime_type -i ^video/x-ms-asf$
acl audio_video_request req_mime_type -i ^application/vnd.ms.wms-hdr.asfv1$
acl audio_video_request req_mime_type -i ^application/x-mms-framed$
acl audio_video_request req_mime_type -i ^audio/x-pn-realaudio$
acl audio_video_reply rep_mime_type -i ^application/x-shockwave-flash$
acl audio_video_reply rep_mime_type -i ^video/x-ms-asf$
acl audio_video_reply rep_mime_type -i ^application/vnd.ms.wms-hdr.asfv1$
acl audio_video_reply rep_mime_type -i ^application/x-mms-framed$
acl audio_video_reply rep_mime_type -i ^audio/x-pn-realaudio$
Regards, Vignesh K vikkymoorthy@yahoo.com