Or he was in a hurry and didn't want to show back up wherever with full tanks and decided to be funny. I've heard of pilots dumping their load and telling the farmer they dusted. If this is a kind of salt-water like they are saying, the farmer probably would never know.
Either way the guy needs to be shown the errors of his ways, perhaps an hour or two alone with the people he sprayed.
Send him to the FBI, they are still "considering" the people's justice.