I live with my daughter and her family in my own basement area consisting of bedroom, living room and bathroom. I have overhead vents for the AC, and with the basement usually being cool, when the AC runs I am freezing. I run a space heater to compensate but is there any other solution?
If there are individual dampers close them down as other have suggested but also block off part of the return air duct.
these normally do not have dampers so a handy trick is to use plastic wrap , cling wrap and block part of the return. Aluminum foil will also work.
You can close the vents totally off (not a good idea) and the intake will pull the cold air downstairs so reducing the intake helps by not aiding the cool air dropping on top of basements are naturally cooler.
Have them open the upstairs vents and that will pump more cold air up stairs. Now you will need to reverse some of this for winter.
When I put in multi floor systems I put in duct dampers to do this at 1 point for each level check you may have duct dampers and then once adjusted you may only need to block an intake to eliminate the heater.