For clarity. These are on my lowest floor of 3 in a townhouse. As such this floor ends up being 4 Celsius colder than upstairs when trying to cool the upstairs to the ideal temp. If I could divert some of the cool air by blocking a vent or two, it would help keep a more even temp on all floors

Understand the issue. Sometimes these work but if they do you usually get lost of whooshing/whistling from the increased air velocity at the register. If you have dampers at the trunk to branch duct connection, use those to alter flow. If you don't have dampers, stuffing some batting loosely into the the boot (the sheet metal thing under the register) until you get the desired reduction in airflow.

The real solution is to see if the ductwork need re-working and/or get dampers installed that you can adjust every spring and fall.

Yeah I fixed one of these 3 story townhomes years ago, been back there a year ago to fix other HVAC company hackery. As this home owner is outside my primary service area so I charge extra to go there. More drive time = more money. Meaning the more work I have to do, I charge for it.

These are extreme kind of jobs. Not many can solve these kinds of problems. A solution meaning: it doesn't create new problems. The solution works, regardless what kind of OAT you are dealing with. (OAT = outdoor ambient temperature)

If you can imagine, how I went back there last year was that the guy they called couldn't fix it and recommended that the person that designed the HVAC zone system come back to fix it. (Me-- I hadn't been there in several years --- worked under all conditions --- hot, cold, mild etc. My climate needs air conditioning 9-10 months of the year.)

In many cases the greater the load (temperature) the more problems there are to "deal" with.

You lack "control" that is a design issue. You won't fix design issues trying to do what you're doing. You must change the design... or redesign the system.

Where is the HVAC system located in your house? (which floor or is it in attic or basement?)

I service the Katy, Texas area.

I would try applying tape to the backside of the grille. The clear packing tape would stop the air flow. If you want a tape that is easy to peel off then you can try the blue painter's tape. Blocking off the vents may help. I suggest you block one at time to and monitor the improvement.

Another strategy is to try to pull more air to the upper floor. Take a look at Quiet Register Booster Fan with Thermostat Control on Amazon. Another poster recently tried this with some success. I usually don't recommend booster fans that are installed at the beginning of undersized duct work. But this type is installed at the end of the duct run. It may work well for cooling. I am not sure how if it would work as well for heating.

The magnetic vent covers are a great idea. I wish I thought of that.