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.
I would go further and make them public as on Kbin.
They already are there, anyway (instance admins can see th), we could as well have them visible