Paul, simple plastic clamp like this [50¢- $1] slipped on appropriate size clear plastic tubing, squeezing the clamp causes it to lock shut, crimping the tubing to stop the flow, very common in the home brew field
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
