Under the New York Department of Labour code section 4.1.13, it stipulates that any person attending a boiler must be sufficiently trained and logs must be kept. Under no circumstances should residents be doing this for safety and security reasons. In addition, what you may consider too hot or too cold is acceptable to other residents. Complaints or requests to adjust the length of the heating cycle or heat should be made to the superintendent or Property Manager.

In addition to the board disallowing access by residents, they should change the key so that only authorized personnel have access to the boiler room.

Just curious - how many apartments and how many shareholders had keys?

2 apts, the warmest and the coldest

You don't say how many units are in the building, hot water or steam heat (1 pipe or 2 pipe), how many floors, units per floor. If there is a disparity of more than 5 or 6 degrees between units, the system may need to be 'balanced'. On a steam system, radiators usually have a vent that can be easily changed to allow more or less air to escape the radiator as the steam comes up, speeding up or slowing down the steam (heat) to the unit. There are also replacement thermostatic valves for the steam vents that will only allow steam to enter if the individual radiator thermostat calls for heat, and the steam is coming up. Depending on the location of the boiler, those units farthest away, both vertically and horizontally, usually need larger vents and the closest ones need smaller vents. Check out gorton-valves.com for general vent info, and also search on thermostatic radiator vents. Thermostatic valves are also available but they will usually require a plumber to install.
Hot water heating systems should have one or more zones for heat that can be individually controlled via thermostats in each zone/area.
Only the super and maybe a trained board member should have access to the controls.

I could not agree more with Ned's comments. Unless someone has been sufficiently trained (and in some locations, licensed/certified as well), they should be kept far away from the boiler and its controls. Even if they "think" they know what they are doing or have been doing it for a long time, your boiler is not something lay people should be adjusting. Change the locks and make sure only your super and property manager have the new key.