Where's the proof? You can't definitively prove it based in what people say and what you seen a photo. Even most digital photos aren't admissible in court cause you can photoshop them.
I'm sure everyone has heard at least one story about a guy catching a bigger fish in an area where he actually caught it somewhere else, it goes with boasting.
You've reported it, the least they can do is go out and babysit the area and keep and eye out for him, it's usually the case.