Comment by solardev

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 for recipients, no matter what the sender wants.

However, if they're using some other carrier or rolling their own VOIP setup, etc., or sending from a toll-free number instead of a shortcode, there's no guarantee that their particular platform will honor STOP. And there's no way for you, as a recipient, to know which is which.

Generally I will reply STOP if it's something I know I signed up for but no longer want. Things I never signed up for just get reported as spam and I don't reply.