Isn't there a pin you can short out which will disable the cpu?
Best 386 Motherboard?
"Depending on the revision of your AMD 386DX, you can completely disable the CPU by setting the FLT# pin to ground. Luckily, the FLT# is next to a GND pin, and you can desolder the FLT# pin and bend it over such that it is touching the GND pin. Turn on your soldering iron and reflow the solder of the GND. Once you float the AMD 386DX, you can solder on a PGA-132 socket and run any CPU. You may not have to do the FLT# to GND solder procedure if your motherboard has a jumper to float the onboard CPU."
You might get away with some foil on some scotch tape if you can cut it fine enough and have steady enough hands and are confident with what you are doing.
I would have thought there would be a bios option or a jumper.
Do you have a mobo manual or the make and model of the mobo or any identifying parts.