If you don't have an internet connection at the site. Wire a 12vdc SPDT relay, normally closed contacts in series with the hot wire of the computer power cord. Use the Arc16 to open the relay. This assuming that the computer is set up to restart on power application.
Please put the thing on a box or on a panel prominently marked as "DANGER EXPOSED 120VAC". There are commercial boxes that do this from a simple closure but I don't have any links handy.
If you have a reliable static IP internet connection, an internet power switch is the hot ticket.