I would choose a different plant like giant burreed. Cattails spread by seed. I would cut off all the seed heads early while phasing out the cattails altogether. Until an alternative establishes, you will have lots of volunteer cattails. Although I have never planted cattails, I rogue a lot out when starting a new pond just from seeds that float in.
Just to clarify, cattails are rhizomes that spread to distant sites by seed, but spread locally through their extensive rooted network. To definitively kill, physically remove the plants, roots and all, or use Esshup's recommended mix to poison them (send him a PM).
