Hmm, in digital devices something is either 0 (low/false) or 1 (high/true). It cannot be both, and it cannot be neither. However, if you don't want to output anything at all on a pin, set the pin as an input. That way the arduino won't drive the connecting pin. This means some other device can set the pin level. If no device drives the pin, its state is undefined (it can be anything and everything). Digital things don't like undefined (undefined is not a number), so you may want to avoid that last situation.
Pieter