What can you do if you suspect a neighbor is poisoning your pet?

Our neighbor is a belligerent old man who seems to find whatever he can in the neighborhood covenants to har us with since he has nothing better to do. Today he's mentioned he's upset with our cat going in his yard(although there are at least 6 cats in houses on our cul-de-sac) Our cat has been vomitting more than usual, and I didn't know if there's anything that can be done besides keeping the cat inside all the time.