If I stop casting, the program carries on unabated on the TV!
If I close down the browser the program still continues. If I shut down the PC the program is still being shown on the TV.
The only was I could stop the program being shown was to disconnect /reconnect the router.
I then experimented with the Android All4 App on my Nexus tablet. Exactly the same thing happens. Once the program has started you are stuck with it unless you take drastic action.