Tell him he is never welcome back and if he ever comes back his ass will be raw with buckshot. He should get the message.
Only good side to this is he probably didn't wipe out your pond but what he did was bad enough.
I would however send him a bill for the dead fish and indicate if he doesn't pay up you will see him in small claims court, which you should do if he doesn't pay up. Don't be bashful!
