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.