This shows you how to access the flash switch. From there should be just a matter of running wires to a connection you can install.
If you wire from the switch in the shutter, then simply turning off the Holga flash will disable.
This year Holga Week crept up on me. In the end, I had one film and only one chance to use it. I wish it was during the summer, not when it is getting darker in the evenings. I also had other issue…
If you do it please post up your your fix. Wouldn't mind doing it myself.