First off lets just say, your friends are a bunch of cry babies ;Þ
Kombucha is a natural product, and the result of a natural product is additional growth.
To stop this growth, you'd need to kill the microbes causing it. If you do that, it almost defeats the purpose.
One way to do this is to pasteurize the Kombucha Tea, either before bottling, or after. But if you do that, you'll kill the yeasts that make the CO2. So since you like the bubbles, you'd have to force carb instead. There are aerobic (with air) and anaerobic (without air) bacteria in KT. Both are capable of producing pellicle (SCOBY), but the aerobic varieties do it much faster.
Ginger seems to boost carbonation (yeast), and kill bacteria. I don't use ginger, so other than what I just said, I have no further empirical evidence for you.
2¢ - Instead of crying about it, maybe your friends should consider themselves grateful that you can provide them such a beneficial and "living" product. They should instead be concerned if there was no growth.