No, STOP is a keyword that the carrier intercepts. The recipient never sees it. The carrier will ensure that you don't get any messages on that number anymore. The business behind the number can (sometimes, depending on the platform they're using) set a custom reply to STOP, which is what makes it seem like they're getting the message, but they aren't.
Of course, it only applies to that phone number, so they can still spam you from another number in their rotation.
Of course, it only applies to that phone number, so they can still spam you from another number in their rotation.