Do you know him? If so when you see him again, say you'll report him to the police as being a peeping tom..his word against yours. If you dont know him, take his photo, then say you will be taing it to the police to report him as a peeping tom.
I think he has got some b*lls coming onto someone elses land to steal from them. Surely it is theft if he takes something which doesnt belong to him. After all, mushrooms do have a monetary value do they not?