the word is abstention.
if you think that voting will have a positive impact, then vote. it is possible for voting (for anything, really) to be both positive and negative at the same time.
the idea that voting "none of the above" would do anything at all (send any message to anyone, much less anyone who cares) seems hopelessly naive to me, but i'm old and cynical, so i'll just mutter away here in the corner...
finally, voting is NEVER our only defense.