Cattails also use vegetative propogation to expand their presence.
This is achieved through stolens that radiate from the primary plant's root-system.
It might be possible to physically confine cattails by encircling an established patch with a root-proof barrier that penetrates several inches (12"?) into the hydrosoil (coupled with the practice of catkin clipping mentioned above). Sounds like a lot of work to me.
Another option is to cut 30-gal or 50-gal plastic drums in half; fill them with appropriate soil-material and bury them almost up to their edge in the pond's bottom. Then, sprig some cattails within the buried vessels - which should keep most stolen-growth contained. If any seedlings emerge outside of the drum-planters, pull or treat them before they become established.
