sure you can cut them off and treat the stubs. how much brush do you want to have stay in the pond? after you kill the main trees, they will eventually collapse into your pond which you may or may not want, but submerged brush does make great habitat.
i have had good success on all my unwanted tree species by either girdling or just heading off and saturating the fresh cut stubs with the ortho (full strength - not diluted).
