Apartment building's thermostat located in unheated common area is cooking the tenants
I am assuming you own the building and can make these changes, but depending on the amount of money you want to spend:


Turn down the temperature on the thermostat
Move the thermostat to one of the apartments, probably in a locked box so the tenants can't actually control it, but it at least reflects the real temperature.  You can also get thermostats with remote temperature sensors, in which case you could install the sensor in the apartment
Install a radiator in the common area, so it at least somewhat reflects the temperature of the apartments
Install a multi-zone system, with a thermostat in each apartment and independent solenoids that control heat to each apartment. This is obviously the most invasive and costly option, but would put individual control to each apartment and possibly reduce your overall heating costs. Using programmable thermostats would also help, as it could lower the temperature at night and possibly during the day while no one is home. The only problem with this is you can probably only do it with individual tenant control, and if the tenant is not paying for the heating costs they don't really have any incentive to lower the temp at any point (unless they like it colder at night).