you could bolt a peice of plywood to the computer, at an angle. kinda like this (excuse the crued drawing), that way you woudn't be able to get your hand in to unplug the cable.
Not verry pretty, but it should work, if you use wood screws to screw it down to the table, and bolt it to the computer case, that should deter them enough not to take it. I don't know how your cases are attached so bolting it might not be possible. but i think you get the jist of my idea.
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Whats the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons, civil engineers build targets
