It kinda depends on which platform handles their bulk messages. For example, if they are messaging you through Twilio, replying with "STOP" will cause Twilio itself to opt you out of messages ( https://help.twilio.com/articles/223134027-Twilio-support-fo... ), and the sender can't disable that ( https://help.twilio.com/articles/360034798533-Getting-Starte... ). It's kinda like how Mailchimp handles unsubscriptions fo…
Speaking mostly for the US here. The STOP keyword is mandated as unsubscribe at the carrier level (Verizon, ATT, TMo) not just the vendor level. So if you reply STOP, it's very likely that you will not receive another message from that number. This will be true for any programmatic SMS vendor. There could be smaller scale & more manual approaches, but that would be rare. There has been a big effort in the last year+…
I inadvertently replied to one of those spam messages because the number coincidentally matched the location a relative had recently moved to (I figured it was them trying to joke around while informing me of their new number, it was something along the lines of "Can you guess who I am?"). They replied with a picture of a girl and some question trying to start a conversation. So, I figure they're just fishing for easily tricked or lonely people to manipulate into sending them money.
FWIW I didn't notice much of an uptick in scam texts/calls after that.