How to make a PowerSwitch Tail
It looks like they used to sell a kit. There is a schematic near the bottom. A relay of some sort is your best bet. Just be sure it can handle the current and voltage requirements. Also, be mindful of how much current the relay coil will need to operate (unless you use a solid state relay). You may need a transistor in between the Arduino and relay to amplify more current.

As always, be extremely careful when dealing with high voltages and current. Do no undertake this project until you have done thorough research of what safety precautions you need to take.