All you need to do to keep them from flowering is interrupt the dark phase. Then, when you're ready to let them flower, stop interrupting the dark phase. You would literally need a computer-controlled set of lights to do what you want to otherwise.
If you drop the daylight lighting phase by as much as one hour right now, they're going to flower. Try it with one or two of them. JUST interrupt the dark phase, you'll keep them vegging. 1-2hrs is sufficient.
Read my thread "Why are my leaves growing three fingers? Let me tell you why" and you'll learn what will happen if you just change to that photoperiod. Or, just do it and see how it goes. Then read the thread to figure out where to go from that point on. :)