A mechanical float value and automatic shut off valve does the trick. Have you never heard of them? The two will cost you around $50 and are sold virtually everywhere that sells RO/DI systems. Here is what J&L carries:
http://www.jlaquatics.com/aqua-fx-fl...t-off-kit.html Electric float switches and solenoid systems can also be built if you're into that sort of thing. I personally put more faith in the manual type.