You won't unless you sterilize the hell out of it and assume they aren't, or until there are so many bacteria on it that it becomes visible as splotchy and moldy spots (then it is too late).
Boiling it for a while does a good job of making it sterile, then prepare something like tupperware and sterilize it with some cleaning alcohol or something like that, and fill the tupperware while still hot. Let it cool down to room temperature. (This part would be like storing jam).
Then if you stick it in the freezer there is minimal contamination. The freezer only pauses contaminations by bacteria or fungus or slows it down a lot, it doesn't kill them - well most of them survive anyway. We should realize from food freezing that it doesn't keep things indefinitely.
I guess for extended storage it would be possible to occasionally thaw the tea and give it a boil again. But the better the boiling sterilizes it in the first place, the more redundant that would be.
Re-boiling could be okay to minimize germs but like dg420 said, if you haven't stored the tea in the fridge it is past the germ stage. After germinating, especially when forming visible colonies (this indicates another stage of growth after germination and consolidation because bacteria first prepare for the right time before signaling to each other to "go for it" all at the same time.
Just killing bacterial colonies may not be enough at that point because they can produce toxins that won't get destroyed as easily as the bacteria are killed.
Venrak is right in a way, but it depends how long you plan to store it. And just freezing the tea without treating it any way else, like I said, becomes less of a guarantee with exceedingly long storage periods.
