Trim the tops of the plants that lay across and clump on the surface of the water. The top of the branch should end just below the surface or a few inches below the surface of the water. Excess plant material floating on the surface can block light and oxygen from reaching the waters below.
Thin the bulk of the plant underwater by pruning back individual plant branches or large sections down to the crown of the plant just above where it attaches to the soil.
Reduce the abundance of the plant more dramatically by pulling it out by the roots at the soil line in large clumps.
Carefully collect all of the plant the cuttings from the water and discard them. Leaving the cuttings to decay in the water will foul the water and breed bacterial growth.