What if the downvote button was only available inside the snap itself. It wouldn't prevent legitimate downvoting, but it would prevent people from just downvoting from the frontpage.

It would make downvoting more of an effort, which won't stop ideologues but would discourage the casual troll. It's been suggested that there should be fairness: upvotes should be given the same restrictions as downvotes but I don't know that I agree with that. Not voting should be all the commentary needed if a snap or comment isn't worth your time.