Hello!
Why would you want that? It would deprive you of possible recording opportunities!
Some time ago, my girlfriend wondered why our box was switching to another programme and she couldn't switch back. A second recording had been started and both tuners where occupied.
I don't know how VDR handles this situation, but I liked what my old box, Gigaset M740AV, did: It asked the user for advice (not start the recording, stop one of the running recordings or switch live channel), and if he didn't react (so obviously no-one cared to watch live TV), it switched so it could do the recording.
Ciao, Eike