The 5V DC supplied trough the cable is what tells the auto-switch to switch over to the appliance last turned on.
Regarding turning off HDMI, you just need to cut a single wire in the video link and that disables the HDMI link alltogether.
Remember that HDMI are very highspeed signals, so using an analog switch IC will add too much capacitance, so that won't work.
Use a small DIL Reed-relay to cut one of the data wires. Keep all wiring very short to avoid problems!
// Per.