Can you prove it?? Maybe. First, quickly develop the other roll of film and see if it got his picture. Second, Start calling all of the film developers to see if there is any film for "what's his name". If you don't know his name or don't want to try that approach, maybe you could let them know what happened and ask them to notify you if they develop a roll of deer pictures. Keep checking, it may take a while for him to have them developed, but I bet he will sooner or later.

This kind of thing sure sucks. I was out scouting a couple years ago with a $400 camcorder(hoping to video some deer), walking down an old lane through some woods. I was supposed to be the only one with permission to be there. A deer trail cut across the road through some extremely thick brush. I promised my wife I would not tear up her camcorder, but I really wanted to follow this trail a ways. Thinking there was noone within miles, I sat the camera down in the lane so it would not get scuffed up, then followed the trail about 100 yards. I turned back and went to the old logging trail and the camcorder was gone!! I had only been gone 10 minutes! Some lowlife was watching me and grabbed the camcorder as soon as I got out of sight! A lot of losers out there, that's all I know.
The good news: There are people out there who will do the same with your truck,your dogs, your 4 wheeler, treestands, even your kids!!(MORONS!). Just be glad it was a $65 dollar trail camera instead!!!

Good luck, I hope you can get it back somehow!