there is actually a funny way to shut down windfarms near forests (although that would probably only work at night/dusk/dawn).

Just blast with full power a recording of ultrasonic bat noises to the turbines.

But as others pointed out, if you have physical access, of course you'll be able to do something nefarious.

Then, wind farms are offline all the time, for all different reasons, so this would actually not that big a deal (apart from the owner of course). And that you are able to hack into the protocols, it is not really surprising, since most of the stuff has no authentication at all. You could of course filter for the MAC addresses of the devices, but this is so much hassle for any maintenance personnel that I doubt that it is in place in that many places.

Additionally, at least in Europe, most of the wind farms are rather small, so we are talking about a few turbines at once. The large ones have actually often enough dedicated staff which will look after them.