This is very likely already illegal, depending on where you live... It violates reasonable expectations of privacy, as well as nuisance laws in many jurisdictio

This is very likely already illegal, depending on where you live... It violates reasonable expectations of privacy, as well as nuisance laws in many jurisdictions, and can probably be considered trespassing. Not to mention they'd need to be flying the drone within line of sight, and depending on the weight of the drone they may also need a license and spotter. Step 1 should always be to go talk to your neighbor before escalating further, but in cases like this it seems like it would be easy enough to have the police pay them a visit too.