[Crito]

Birdshot. Range would be an issue, but the higher up the target is the more margin of error you have.

That or maybe falcons. Train birds of prey to knock drones out of the sky. Might not be safe for the bird, depending on the type of drone. I wonder if falcons are smart enough to make that sort of judgement call themselves..

[lttlrck]

They could be trained to drop weighted rope onto the props.

Edit: though I guess there might be some simple countermeasures