Privacy fence with lots of thorny bushes, rose bushes, planted beneath so folks can't/won't jump it. Put a lock on the inside and don't rely on the gate lock that will be built in. If you like roses, rose bushes work really well for this purpose; but there are other plants that will get the job done. So too, barbed wire rolled beneath the fence if you can stand the unesthetic aspect of it and potential liability if someone gets hurt jumping the fence. Bushes are the better option if they will grow in your climate and you have room for them.