The airport knob is the thing that you have to turn out to insert your air tank and turn back in to settle in you air tank.
you cant stop it but there is a chance that you are missing a o ring on your co2/ n02 tank. after that your leeking should be over
As stated, check your o-ring...may be worn and needs to be replaced...very easy to do.
That would be your ASA, and make sure your bottle o ring doesn't have any rips and is in good conditon, if that is fine try putting an o-ring inside your asa and screwing your tank in