I think installing one of those aircraft landing lights would have been a better response. Just hook it up to an incoming light sensor and a day/night sensor, so anytime the neighbor turns on his flood light he can't see a thing down his driveway because of the blinding landing light.

No need to trespass, send emails, or even talk to the neighbor who will soon discover that every time he turns on his floodlight his driveway and front yard become uninhabitable.