I'm on an IT team in my company and I would like to block youtube to users. I don't want to just deny access to the whole youtube domain, but only to replace the .flv/.mp4 request with the one that I want. That way, if someone tries to watch youtube videos on the network, He'll get a video of why using our expensive bandwidth for pleasure is a no-no.
I thought about using a packet manipulation program and just replace the video ID with something that I want, but I didn't manage to do it right.