I think this is an effect from buyers sitting behind their phones/computers/tablets and not getting any negative reinforcement when the seller turns them down. Like most of you said, you either don't respond to lowball offers or you just respond with "no". They'll just keep fishing on a ton of ads until they get something for a stupidly low price.
It's morally hard for a buyer to lowball a seller when you're speaking to them in person. Which is why I write in my ads "I will only negotiate the price in person." This will turn off some buyers since they want the price figured out before they arrive, but it also helps filter out who's serious and who is just fishing. I've never felt the need to tell someone to pound sand of F*** off yet.