I used to visit a park that had catnip growing wild. On a nice summer day, there were slightly crazed looking cats everywhere, after the catnip. They didn't look like feral cats, and there were no houses closer than 1000 feet away. A fenced box, as the first poster suggested, is the only thing guaranteed to work. If you just put a fence around it, they'll only jump over unless it's at least six feet tall. It had also better be anchored to the ground, or they'll tip it over and get under it. A sort of wire cage with mesh small enough that they can't get their heads through, small enough to protect the main part of the plant but large enough so a few stalks can poke through, will work fine.