My Siberian irises are too tall. I live in northern Florida and they are the same all year long, so not sure how to trim them back?
There is really nothing that you can do to reduce the height of your Iris stalks. You can stake them if they tend to fall over.
Hybridizers do try to produce plants that grow a sturdy stalk. There are also many cultivators that are smaller or even dwarf varieties.
You can dead head the flowers after they are spent and cut down the flower stalks after they have completed blooming.