PORKY08

Afraid not, once they're put into flower the stage should already have been set....

I'm afraid not. All you can do to slow their growth down, which I'm presuming you're wanting to do, is keep the light hours just above flowering and cut down the nute strength. Less energy means less growth.
(Unless you meant buds, not clones?

Topping with LST is a good plan.
Topping first will help keep the height down loads, then use your LST to guide the branches around. You can even top a second time, or individual branches, whatever you need/wish. It's no big ask to get the height down, but the fun begins in training a nice even canopy, but it's always nice to have little jobs to do for itchy newbie fingers!
And topping will delay growth a week or so, but LST has little effect on slowing growth rate.