The only way to eliminate auto voting is to reduce curation rewards if you constantly vote on the same author. For instance, you could add a curation multiplier (0-1) that is associated with time. If you haven't voted on an author for say 5 days your curation is multiplied by 1. If you have voted on the same author 10 times that day your multiplier is 0. The difference is burned or donated to the HPS.

Does that make sense? It overly complicated things, but gives you the voting behavior you are looking for. Although vote bots could be setup to get around that. That is always the case.