Cattails can be killed using both mechanical and chemical means. New growth that has grown approximately 6 inches above the water's surface can be pulled; the entire plant, including the root, must be removed to ensure that it doesn't return. A different mechanical method requires cutting the tops off of new shoots as they emerge. This method deprives the roots of food, causing the plant to wither and die.
Chemical methods that are used to kill cattails are often easier to maintain although certain chemicals must be used to prevent the contamination of the water housing the plants. Two chemicals that are safe for aquatic use when killing cattails are diquat and glyphosate. Diquat is a contact herbicide. This means that the chemical only kills the parts of the plant that it touches. The disadvantage of using diquat is that it doesn't travel through the plant, so it doesn't kill the root. Because the root is still intact, the plant can continue to grow new shoots the next year.
Glyphosate is a systemic herbicide. This means the chemical travels throughout the plant killing both the stems and root. Unlike diquat, there is no need to spray around the plant from multiple directions to ensure that the treatment is effective. Simply spray the portions of the plant that can be reached easily. Typically, only one application is necessary to kill stands of cattails.