what size duct serves the room over the garage?
what is the capacity of the hvac system?
can the ducts in use handle the additional
air flow or are they undersized for more air flow?

there is more to it than just disconneting
a duct.

the guy that suggested this..he is hvac guy?

best of luck.

You could do it by simply blocking the registers in the room. If the duct feeding the room as a damper control you could close it to block the air.

Have you looked into adding more insulation to your attic? The increase in insulation would lower the heat load and allow the AC to keep up.

if the damper is at the plenum you would be
putting the addtional air back into the plenum
to be distributed through other ducts.
which may not be sized to handle additional air.

just shutting the register doesn't put the
air back into the plenum. it does little
to distribute the air throughout the house.

there is a reason that there are manuals for
both duct design and sizing.

adding insulation on the attic floor does nothing
to the heat gain through the walls.

putting foam/foil sheathing on exterior of walls
shared with attic will stop both air leakage
and heat transfer.

foam sheathing will air seal when installed and caulked
in place. foil facing into the attic will reflect
heat back into the attic and out of the walls of the
room.

rooms over garages are surrounded by hot attic
air. this air gets to 140 degrees in summer.
air sealing the walls works. otherwise you
are just pumping air into a room with extreme
temps.

best of luck.