Anyone with half a brain could spoof ACARS messages, this has been discussed before. These scenarios often get a lot of attention because it's "computers", but essentially it's the same as a goober with a 100 USD handeld VHF who interferes with voice communication. Annoying, and a crime, but all in all not the biggest threat to aviation safety in the real world, as pilots are flying the aircraft, not ACARS...Agreed with G0ULI.
The "scariest" scenario I've seen so far was researchers feeding false GPS signals into a ship, affecting it's autopilot, but that was not quite trivial to achieve and required some expertise. Researchers have also successfully interfered with TCAS, but a false RA could only be generated under very rare circumstances. In the end, there's a reason why pilots are flying the aircraft.