My neighbor is spraying chemicals in his lawn.

Lawn treatments can be toxic to Koi.

If your pond is more than 20 feet away from your property line, you shouldn't need to worry about the chemicals that your neighbor is using getting into your Koi pond.

If your pond is closer than 20 feet from your property line, cover it with clear poly-plastic.  Use some 2x4s, 4x4s, or PVC pipe to keep the plastic raised above the pond.

Perform a complete water change within 24 hours to remove or lower the concentration any stray chemicals that do end up in your pond.

The edges of your pond should be raised to prevent rainwater from picking up chemicals in the yard and depositing them in your pond.