Have you checked for water contamination downstream from your pond? perhaps if you had other neighbors aware and concerned with what is happening to the ecology in your area there could be more action taken.
It sounds like you have taken the proper steps to try and first ask the neighbor and seek agency help when that didn't work, but I would probably have to swallow my pride a bit and try and to set a face to face meeting with the neighbor and see if you can't successfully plead your case while also nicely insinuating that you have received clear indication from agencies that he is in violation and that you don't want to BUT WILL sue him resulting in high costs if you two can't reach agreement.
If he is a horses ass that you already know is not going to respond to that approach, I would write a blog, or have someone you know write about the issue and distribute it to as many area newspapers as you can to see if someone will publish it. Area ecology groups may also want to take up your cause and get publicity to the issue. When your neighbor starts getting funny looks at the grocery store from the people in your area he probably will fix the error of his ways.
