Unfortunately, there is no law or official way you can make them stop.
We advise that you contact that farmer directly. Your council will not have the authority to dictate what an individual does on their private land. It is the farmer’s discretion to decide what pesticides to apply and when to apply them. They should be keeping records of the products they apply as best practice. You may be able to reach an agreement so that the farmer notifies you and the community before they apply pesticides. Some farmers have used a lag system to notify neighbours of when they have ben applying, so that neighbours can take precautions i.e. close windows, not go walking with children or pets through sprayed farmland.
Ultimately, the only way we can truly change the use of pesticides on farmland is through government policy and by calling on farming organisations and businesses to reduce their pesticide use.