How do I disable a thermostat to prevent it from turning on the AC?
It's going to be the Yellow wire that calls for cooling (surprised that's an X and not a Y).

Two things to be careful of. First, the O/B is for the heat pump valve (switching the compressor from heating and cooling modes), so I'm not familiar enough with heat pumps to know if activating this valve but not the pump itself could damage anything. I don't see why it would, but it's something to consider.

Second, if you are running an AC when it's below 60F, you run the risk of freezing the coils (literally, the whole thing could be covered in ice if it's running at this temperature). Ice on these parts can cause permanent damage, in addition to being a waste of electricity since air isn't going to pass through a block of ice. If your roommate still thinks this is a good idea, now may be a good time for them to sign a liability waver to cover the cost of replacing the AC if it becomes damaged, and to switch the electric bill into their name.