You can kill any process in a script by calling the script below, however if you want to stop Runtime cleanly, on the Toolbox, on the Runtime tab, there is a static method you may add to an automation and call labelled "RuntimeHost.TerminateRuntime". This will trigger the shutdown of Runtime as if it were exited from the tray icon.

foreach (var process in Process.GetProcessesByName("calc"))
{
    process.Kill();
}