RLewis,
One possibility is a fish trap. I have caught snakes in them that have drowned trying to eat the fish in the traps.
Another possibility is to just bug them to death. Snakes don't like to be disturbed and by throwing rocks or whatever every time he shows his head above water, you may drive it off.
A 22 pistol loaded with bird shot does not make much noise or carry very far. Probably not advisable but if I were desparate...
Your city may have an animal control unit that would dispatch the snake, but unlikely.
As for prevention, harassment is the only thing I can think of to suggest.
