Since we're now living in the age of the internet, you can't expect spacecraft to stay in the solar system. To blame Sark for that is just plain silly.
When it comes to our solar system, the "fence" just doesn't matter anymore. With FaceTime, Skype, email, and teleportation; Sark is going to recruit the best aliens on his bored, regardless of solar system. And he's not doing it by burning bridges on Jupiter, Saturn, or any of the other bigger planets. You guys just need to think about it and stop stomping your moon boots.