Ftfy.
Unfortunately many sms spammers will only see that "stop" as confirmation their messages are being read by somebody just like a lot of spam callers do
Still not sure at least in the US or EU that spammers could exist for any useful amount of time. The Carrier knows it is a spam machine, they will block it. Unlike an email account the carrier makes money on subscribers and needs access to competitors networks to provide coverage.
Scenario one is a rouge carrier or some virtual carrier just allows this crap to continue, the people who own the “pipes” will shut them off. This has a monetary cost, not sure Howe much but it won’t be cheap.
Scenario two, a user on a big carriers network. That user will be shut off. These are all easy to do things. Yeah the person who gets shut off will get a few warning and yeah they could game the system by buying prepaid sims but again there is cost to this stuff. $30 worth of prepaid sims better earn at minimum $30 worth of business from the spam sms.