If it is FF CNG, it has FF Air blocker fitted in the air filter box. It opens the air flow during operation on petrol and blocks during operation on CNG (It has two wires connected to it). Just extract that device and install it anywhere before the throttle body and get it wired.
If it is not FF setup, then try to get that device from Kabarr market and install it. If you cannot manage to get it, then take to a CNG tuner to block the air.