Yeah this was mind blowing when I first moved to the US.
The idea that someone can cost you money without your consent is insane. I vaguely recall incoming calls also costing money when I first came here - and I got a lot as the previous owner of the number clearly hadn’t been paying bills...
US Telecom and Banking industries are very primitive compared to many countries, so no you cannot do much. Sad but reality. You can either pay for unlimited text or pay for each SMS.
Unlike E-Mail and landline phones, unsolicited calls and messages on mobiles and in regular mail are not illegal. Perhaps because both USPS and AT&T make a decent amount of cash off this.
For automated texts, legally, there have to be ways of unsubscribing. Try replying to them with "STOP," as even if the spammer doesn't configure it, usually the upstream SMS provider has it built-in to remove you from getting further texts.
In general, essentially everyone in the US has an unlimited message plan, and if not, many carriers don't charge for incoming. My recommendation to how to fix this is to vote with your wallet. Switch to a prepaid provider with better SMS rules.
I'm assuming based on the $0.20/message cost, that you're using AT&T prepaid, which has some of the highest SMS fees for prepaid plans.