You tell the ACARS system not to use them.
That would be more accurate.
What I’m hung up on is that the ACARS system was turned off while still over land in Malaysia, before the pilot(copilot or hijacker) acknowledged ATC verbally before entering Vietnamese airspace.
Some basic information that I would find helpful, would be if they would allow some friends of the pilot and copilot to listen to the acknowledgment to ATC, in order to potentially confirm who was at the controls at that time.