Bass,
Without having been in a situation like yours, I really don't have any concrete answers but here is what I would do. First off, invite him fishing for a day. Stick a copy of PondBoss magazine in his mailbox to try and melt the ice.
If that leads to a dead end, I would review a copy of his deed. Hopefully it will contain wording which would clear up the issue of pond ownership and fishing rights. If it does, perhaps you could approach him and talk over the situation. The Commish also offers another approach, buy his rights.
If he still insists on no fishing and again threatens to poison the pond, assuming your homes are serviced by wells, explain to him that the potential exists that both wells might be contaminated by the poison. The chances of this may be remote but it might give him food for thought.
If diplomacy doesn't work and someday you are greeted with a pond full of belly up fish, your neighbor might just find that the pond "flows" in his direction and its a shame he ended up with a shoreline full of stinking rotting fish.
Like Dave mentioned above, you could contact the authorities and seek legal counsel. Do whatever you can to avoid court. With law enforcement and the courts, you'll need proof. I think in your situation it would be hard to get the proof you need. Sure you'll have a pond full of dead fish and you can say the guy told you he was going to do it but, in court, evidence will be your best weapon.
Good luck and keep us posted.
Russ
