>How about a low-cost hardware solution if you don't need to use the
>computer for other things after -hours?  You could put a locking barrier
>over the "on" switch, or maybe cut power to the outlet via the circuit
>breaker.

Most PCs have a key lock next to the rest buttons that can be locked to
prevent the computer from being turned on.  I don't know about locks on
Macs but you can config them to ask for a password before completely booting
up.

Deb Fuller