Unfortunately after testing your theory for a while now, I find that you are not correct. Turning my default rewards to "declined" has absolutely no effect on stopping down-voters.

Here is proof:

https://steemit.com/steemit/@starworld/list-of-steemit-downvoting-scumbags

Notice that the scumbag @block-power (AKA @the-reef) was still able to down-vote and hide my post.