One word, to any of them. It takes effect immediately and nobody will try to talk you out of it.
Tess, Ebz, Jamie and Frankie all stop at once. You do not have to repeat it three more times.
A single message saying it worked. After that, silence. No check-in a week later, no "we miss you", no offer.
Stopping and deleting are different things. Your conversations stay where they are, so if you text START in a year they still know you.
Text START to the same number. It picks up where you left off rather than beginning again from nothing.