Hello and welcome, Elizabeth. My name is ***** ***** I'm a biologist with a special interest in fish. I'm sorry to hear that you're dealing with this problem.
Many areas in California are spraying with a pesticide called Evergreen, and they claim it's safe for fish. However, at the same time they say that, they recommend that humans stay inside with windows closed and air conditioners off. That doesn't sound like they have a lot of confidence in the safety.
Protecting your koi is going to be difficult. Under these circumstances, it is recommended to remove them to tanks or children's wading pools inside a garage, with heavy aeration. The pumps and filters are then removed from the pond and stored inside in containers that have been filled with pond water. This protects the beneficial bacteria. Then the pond is sealed off with heavy duty plastic and duct tape. While this method protects the fish, it also places a great deal of stress on them. This is a complicated method and does pose some risks to the fish.
Another method is to turn off all your equipment, place some AmmoLok or Amquel in the water according to directions on the bottle. Then seal off the pond as described above. This, too poses risks for the fish, as oxygen can be depleted. If you leave the filters running, it will draw in the pesticide.
Finally, the easiest method is to turn everything off, then just cover the pond with plastic, blankets, whatever you have. Again, we have to worry about oxygen depletion. If your house or garage is close enough to the pond to set up a pump inside with airstones connected to it, you could leave that running to provide air.
No method of protection is going to be 100% effective, and all of them leave the koi exposed to some risks. Unfortunately, this is going to be the best you can do under very difficult circumstances. I wish you success in protecting your koi.