acl mimeblock rep_mime_type ^application/x-shockwave-flash$
acl mimeallowurl dstdomain .youtube.com
http_reply_access deny mimeblock !mimeallowurl
to the ACL section in the squid config file. Flash just works as an example here, other mime types can be blocked as well.
http://en.wikipedia.org/wiki/Internet_media_type
