I never downvote a comment, reddit or here, simply because I disgree. I find such behaviour (subjectively) wrong since it does not encourage discussion in an open, civilized manner (on reddit, all that happens is that you get 30 comments deep and you just downvote eachother's comments to 0 while being increasingly aggressive).

I reserve downvotes for when a comment is being needlessly toxic, doesn't contribute to the discussion or otherwise not helpful for an open discussion.

I think the best cure for "downvote to disagree" is to firmly hold to the principle that the opposing side of the argument has the best intentions to the extend of their knowledge and that at the end of the discussion, all participants should have learned something. You should also always be willing to change your mind on what you argue about.

Always.