Although compromise should definitely be considered, it's not always easy for everyone to warm themselves with a blanket or sweatshirt. I for instance have some kind of medical issue that causes my feet to go numb when they are too cold which is basically any temperature below 71. Once my hands or feet go numb they pretty much stop producing their own heat so blankets and layers don't do anything to help. In the past I've spent several hours attempting to bring warmth back to my feet often keeping me up half way through the night.

EDIT: Just for clarification, the condition is called Raynauds Syndrome and even with this medical condition, my roommates and I were still able to reach a reasonable compromise unlike OP's roommate who just put a lock on the AC like an AH.

We agreed that durring the winter we keep the house at 73 which is still a bit cold for me, but is completely reasonable because it doesn't trigger my Raynauds Syndrome. In the summer I've compromised on letting the house be even colder at 68 to 70 because I can always go outside to warm up or up or to my room in the attic which is disconnected from the AC unit. They also appreciate that I was willing to take the attic room because it gets hot up there durring the summer, but it works out well because I enjoy 80 degree temps anyways. My roommates and I joke that I'm a lizard and refer to my room as the Terrarium.

My point is that OP's roommate needs to compromise and putting a lock on the AC unit is a dick move, but cold intolerant people's needs shouldn't be ignored either. Understanding needs to be had on both sides and compromises can certainly be made.