You could try changing your channel number to one that doesn't show on your setup under "receiving from". If you still see your station number after switching, you are picking up another station. I would think that most people do not change it from the default #1.
Other than that, I think that you would need to remove the battery from the ISS and unplug the solar panel. Turn on the #4 DIP switch to enable the diagnostic LED. This will help discharge the super capacitor a bit faster and totally disable the ISS from transmitting after several hours. Once you are finished with this experiment, you will need to remember to turn off the #4 DIP switch as well as reconnecting the solar panel and replacing the battery.
Other than these suggestions, I don't think there is another way to shut off an ISS (or other SIM module transmitter).
Greg H.