when a plant is switched to flower, it usually stretches for 2-3 weeks. after this is over, you will see much less verticle growth and more of a filling out and thickening. basically you need to just wait this period out and not do too much with the plants.
topoing during flowering is a bad idea. it will reduce yield so avoid unless you have no other options. if you need to save a little room, you can tie branches down to lower them a bit. this is also a useful tool when trying to get the buds at the same height