If anyone wants to be EVIL to their Sim, give them the Loves the Outdoors trait. XD
Or even worse, do the mailbox corner, but make the fence a low brick wall. Freedom lies beyond a small barrier they can never step over.
You could probably even just make the entire thing a fenced off area. That's a good question though. Can sims talk to each other if that low fence is in between them? If so, we'll have to mention that you can't use it. At the end of the challenge your friends list should be empty.