Can I manually switch off booster fans that are coupled with HVAC blower?
You'll need to add an extra fan relay to your system to accomplish this

Your existing setup controls the booster fans via the EAC terminals on the furnace control board, which appear to be energized whenever the furnace blower is running.  In order to change this, you'll need to stick a fan relay in line with the control wiring to the fans, so that the fan only comes on during the correct conditions (cooling, in your case).  For this, you'll need:


A SPST-NO HVAC relay (these have 24VAC coils, quick-connect terminals, and flange mounting means)
Two short lengths of 18/2 thermostat cable (enough to get from the control board to the relay, and another shorter length to serve as a pigtail)
A length of 14AWG stranded THHN wire (about the same length as the thermostat wire, can be any color save for white, grey, or green)
Four female quick-connect terminals (two red ones for 18AWG wire, two blue ones for the 14AWG wire), as well as two wirenuts suitable for 4x18AWG wires
A couple of self-drilling sheet metal screws, suitable for attaching the flanges on the relay to the inside of the furnace wiring compartment
And a screwdriver + wire-cutter/stripper (of course) and a ratchet-type crimper with an insulated terminal die (these are the way to go for crimping things, the cheap crimp pliers are not a good way to get uniformly reliable crimps)


The installation goes as follows (if you aren't comfortable with this, any HVAC tech or electrician should be able to do this on your behalf):


Turn power to the furnace off (the service switch will suffice), and open up the wiring compartment of the furnace
Find an empty spot near the control board to mount the control relay at, and use the mounting screws to attach it to the inside of the wiring compartment there
Crimp the red quick-connects onto the wires on one end after stripping the wires on both ends of the longer length of thermostat cable
Strip both ends of both wires on the shorter length of thermostat cable
Crimp the blue quick-connects onto the ends of the 14AWG wire after stripping them as well
Remove the black wire with the quick-connect terminal on it from the EAC-H terminal on the control board, and move it to one of the contact terminals on the new relay
Use the 14AWG jumper you made in step 5 to connect the other contact terminal on the relay to the EAC-H terminal on the control board
Remove both wires from the Y terminal on the control board, and connect one wire in the short length of thermostat cable to the Y terminal.  Take the matching wire in other end of the short length of thermostat cable, and nut it to the existing Y wires as well as to one wire in the long length of thermostat cable
Repeat the process of step 8 with the wires connected to the C terminal on the control board and the free wires in the two lengths of thermostat cable you added. If there's only one wire connected to C, that's no big deal -- this will set you up properly later if you need to run a C wire to a thermostat down the road.
Connect the quick-connects on the other end of the thermostat cable to the coil terminals on the new relay
Button the wiring compartment back up and turn the service-switch back on


Now, the booster fans should only work if the thermostat is calling for air conditioning.  If you want them to run during a call for fan, you'll need to rewire things so that the fan relay's coil is wired between G and C on the control board, instead of between Y and C as it is at the moment.