Or just raise the price on communications, or let recipients set a price on being contacted. Charge $1/text, which the sender gets back if the recipient replie

Or just raise the price on communications, or let recipients set a price on being contacted. Charge $1/text, which the sender gets back if the recipient replies. This would very quickly make most spam uneconomical, since the expected returns wouldn't exceed the cost to send the messages. Spam works because most of these mass communication channels are free or extremely cheap. Raise the price and economics will dictate that most of these communications will drop off.