I imagine you have some kind of HPS, and when they are done streching the light will burn the fuck out of them. I suppose you could introduce a screen (aka Scrog) suspended flat, or V shaped above the canopy. Its way too late really but it acts as a barrier so that nothing goes above that set height. Train all the stretch horizontal to the screen by either weaving it or tieing it. I can't imagine how LST would work at such a late stage to be honest.