Ponds and range
Great question. There are a lot of methods suggested to control both of these pests, but none are easy.
The cattails, Eliminator and Alligare are the top two that are often used that do a great job.
The cactus, if there are only a small amount then physical removal is best but if not possible Surmount is the most recommended with the active ingredient being picloram. The potential problem is that is a restricted chemical so you will need a pesticide license.