There is a reason why you don't have an on off button on a device intended for remote connection. I use a remotely controlled power outlet which I can control over the internet. This way the slingbox can also be rebooted if necessary.
In brief, it is a nexa power switch remotely controlled with a Tellstick inserted in a Dovado router and then controlled with a dovado iphone app designed for controlling the devices supporter by the Tellstick. See Telldus.com and dovado.com. I'm in Sweden but this kind off stuff should be avalable also in the US.
Instead of the Nexa, I recommend GAO/Everflourish for European users, the build is better.
During the spring, a tellstick net will be avalable which can be ip adressed directly over the same LAN that the slingbox is connected to. That device is sold out now.
Or pull the plug
Use a wemo power device that can turn it off, if not in use or use a device call vera http://getvera.com. Remember both devices can be turn off and on with the internet or data from your phone. Hope that helps.