And be sure to never book through the mobile app. As far as I know there's no way to have it not use your telephone number when booking through the mobile app so you will always get text messages. It's one of the reasons I will never use that booking tool.
Also, the website will remember that you don't want your phone number selected when booking a reservation. It's been this way for me for years so I never get a text.
The "STOP" feature is for that reservation only. It's absurd and also not clearly spelled out in the text you're replying to.
-RM