Make only each account’s 5 most recent upvotes effect post rank. In effect this would make vote sellers need to make many more accounts to be useful, they would need to churn upvotes.
Things would move and change more frequently. Vote farming would be more obvious and make countermeasures easier.
At least that is how I imagine it would go.
If there is a predictable algorithm, it can and will be gamed by a bot.
The closest I’ve seen for control of voting is Slashdot: each person gets a limited number of mod points (votes) on a semi random basis. Then there is a meta moderation queue where a second group of semi randomly selected people vote on whether a given set of votes were sensible. If too many meta mods disagree with you, you don’t get as many mod points.
The biggest problem with that system is that it amplifies groupthink.