"I like the net idea - but it needn’t be a net, steel wire would be effective since that would tangle the blades."
Not if the blades are cowled. Plus, being these are likely industrial-grade drones, what's to say their blades aren't metallic and able to cut a wire?
Cowling adds significant weight thus reducing flight time and manoeuvrability. That's mainly why the prop guards on drones are so small vs the total area of the blade (most covering around 1/4-1/3 of the prop's area)
A large ship's propellor can be fouled by a relatively small rope (in this case a mooring line is "relatively small", although even smaller lines have caused big problems). The blade and the engine combined aren't enough to cut the rope. A small engine on a drone will not cut a wire. If you doubt this, get a decently powered lawn mower and see what the same size wire as a top E-string on a guitar will do to it.
Nothing short of a predator drone will have an engine able to cut even a very thin wire.