Sure, purchase a remote power switch that you can either telnet into or dial into. Then you can power the switch off remotely. One company I've used before is at http://www.servertech.com. You can even get "remote console" access (ie: your serial port). Very nice units.
Otherwise, there is no way to shut them down via software that I know of, at least in a way that allows them to come back up easily. You could modify the config-register to have it come up in a non-booting mode, but that would be a pain to recover from I would think.
