Zoned heating system very warm even without heat on upstairs?
Shutting some internal doors might help. Especially the door to the room upstairs with the return vent, to avoid pulling hot air upstairs. Anywhere air can get out upstairs (e.g. bathroom extractors, trickle vents) can encourage convection too. 

If there's heat in the downstairs hallway you might want to restrict it. I have had to do this with our (wet) heating system otherwise the master bedroom gets too hot. Heat from near the bottom of the stairs goes straight up. You may need to improvise, at least for testing. 

Ceiling mounted warm air vents and their ducting have to be well insulated to avoid acting as underfloor heating for the floor above, and yours probably aren't. Feel the floor and you may be able to tell where the ducts run.