First stop would be the municipality, to see if this can be resolved in a non-legal way, second would be the police because flying drones over built up ground is illegal in many places, finally, recording people without their consent is illegal in private space (such as your garden).

I'm not the nicest person in the world and if someone flew a drone over my garden I would ignore it but if they would hover there long enough for me to be irritated I just might attempt to bring it down.

That would at least force the owner to identify themselves and I can afford to replace the drone if ordered to do so by a judge.