This is difficult to answer as I don't know what type of balloon inflator you are using and as I obviously can't watch you ;-)
Have you tried pinching the balloon neck shut before you turn of the valve? When you inflate the balloon, do you use continuous air flow, or short bursts or air, by pressing a stop/go button?
When we inflated our balloons with helium, I found it easier to control with using the stop/go button than with continuous inflation.
If you could give me more details (type of inflator, perhaps a photo), I'll try to help you better.
