Bees swarm, period. Do what you will, they can't all be stopped. Come up with a plan of attack for your area, and follow it. Trying to stop those colonies that are bound and determined to swarm is usually wasteful of time needed elsewhere in the apiary. My plan is fairly simple. It usually works. In northern Vermont, our first major flow is from Dandelion. It can be an intense flow, often plugging the brood nest, and causing swarming. Colonies get too strong before the main flow. Instead of building up on the Dandelion flow, and peaking at the main flow a month later, they're already strong and swarm on Dandelion. At the beginning of the Dandelion bloom, I reverse each colony, and count the number of combs of brood. In my area, five or six is about perfect. With a good queen, and normal weather, they'll be at full strength by the main flow. This is your goal. If they have much more brood than this, like 8 or 10 or 12 combs, they need to be weakened. Remove combs of brood, giving them to colonies who have less than 5. Mark the colonies that are still too strong for dividing when the Dandelion flow starts in ernest. Splitting at this time gives the nuc lots on incomming nectar and pollen. It also ensures the parent colony builds up on the Dandelion flow instead of before it. Get a super on at this time. This takes care of most swarming, most years. Some colonies will swarm anyway. I think it's their way of requeening themselves. I usually let these colonies do their thing. I used to cut cells and fuss, and fuss. The rest of my bee work gets behind, and the bees get ahead of me. When you have lots of colonies, you have to play the averages. Spending all your time on a few colonies just doesn't work. Better to spend your time on supering and requeening.