They become less effective at their job of eating vegetation as they get big and old. Best let the coons have them and buy some replacements when the fish truck comes around.
Bow and arrow. But that kind of goes against not harming them. LOL
Too many in a pond will keep the turbidity up from them rooting around trying to find something to eat.
Keeping the correct amount of grass carp to control weeds yet not muck the pond up is an art. Too few and weed explosion (in some ponds, not all), too many and a muddy yucky brown pond.