Some more modern aircraft indeed have such protections, but this was an old 737 without any such stuff. Simply pushing the control column fully forward would send the plane into a dive pretty quickly.
The controls are mechanically linked. Unless the perpetrator incapacitated the other flier, it’s a test of strength. There’s no method for disconnecting the second set of controls in a 737.
That is incorrect. There is a friction clutch between the controls in case there is a mechanical jam the other pilot can at least control half the control surfaces. It needs a significant amount of force to detach, though. It is documented on Egypt Air 990 however that both elevator surfaces turned into opposite directions.