For a handy list of accounts that have down voted you, go to:
and type in your steem name.
Someone suggested in the comments earlier (i think it was @elisea) to make it that you need to have posted a few times before you get to down vote.. I'd suggest taking it further, much like the vote slider, that it becomes a feature that you get/have earned after having been on steemit for a certain amount of time (maybe 30 days), have a minimum rep .. say 35 and have to have posted a certain amount of times. I know you can get around the rep bit with bidbots but it just makes it that much more expensive for a spammer.. anyway, some kind of mechanic to discourage this none sense would be welcome.