Yellow Flag Iris seed pods start to form after the flower. The pod forms below the flower on the same stalk. If you remove the flower stalk before the pod forms, that should prevent spreading. Many enjoy the flower and wait until it starts to wilt. If the seed pod has not opened, it can be removed safely to keep the lake from becoming clogged with iris.
The stalks with the flower and pod can be disposed in yard waste.
More information is available at https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/bmps/yellow-flag-iris-control.pdf