Reducing the amount of mold spores helps, but certainly doesn't prevent future mold growing since the air is full of spores. Covering it with foil until it starts to show activity may help to reduce airborne contamination.
An active fermentation is really what's needed to prevent mold growth, since it deprives mold of both oxygen and nutrients.
You need to start with a good amount of starter tea, or at least a cup or two of very strong starter. The lower ph will help with keeping infections from starting.
I started my first batch by adding two cups of brewed tea with 3 tablespoons of sugar to a bottle of store bought plain kombucha. Let it sit for a week in and it grew a thin scoby. I dumped the whole thing into a gallon jar and topped off with sweet tea.