It seems like it would be easy enough and cheap enough to build a honeypot to identify accounts used for the purchased Reddit upvotes.
For example, Reddit could set up some honeypot posts to track paid upvote accounts.
They then go and pay these upvoters to upvote the honeypot post and identify the accounts used. (It would be helpful if the post was hidden so other people don't find it accidentally. In fact, it is possible to just use a tracking redirect page given only to the paid upvoters and use any post as the upvote "job" so it would be hard to identify by the upvoters.)
Then Reddit could ghost those identified accounts. Simply ignore their votes in the system, but don't tell the account owners, so the owners continue using the accounts without realizing the problem.
This would make it very difficult for the account owners to know which of their accounts were compromised.
Then on any new posts where these upvoter accounts are being used in majority, other accounts can be found. The other accounts that also similarly upvoted on this article could represent other paid upvote accounts.
Track those other accounts and how often they appear beside the ghosted paid accounts, and voila, you have found more paid upvoters.
Keep doing this and it makes the paid upvoters ineffective because although they can work the system, their work is only being used to find other paid upvote accounts and also clients who are paying for paid upvotes.
After a time period, the clients could be sent a warning:
It has been detected that you are using paid upvote services which are against Reddit TOS. Please contact customer service so we can work together to remedy the problem. Failure to do so may cause your account to be banned and all your posts removed from Reddit. Have a good day.
Of course Reddit doesn't have to do this, and really anyone could do the same process to build a list of paid upvoter accounts and a list of articles and clients that use those services...
So what do you think, would this put a dent in the upvoters effectiveness?
The problem is that many of these accounts are only used once or twice before being retired or sold off. So if you pay the "upvoters" to honeypot the accounts, all you'll do is put a very small dent in their accounts and next week they'll be back up to 100%. And if you pay them every week you'll just become their main source of income.
And the accounts are much more "crafty" than you think. often copying high-karma comments in reposts, or posting semi-nonsensical markov-chain style comments for years before being used as "paid" upvotes.
It's really weird, I follwed one a few months ago. I saw it was posting those markov-chain style comments, and I watched it until it's karma hit like 500 then it deleted all it's previous comments and only had one comment talking about how great some app was on /r/android.
Like a week after that the account was deleted. I'm not sure if it was done by the admins or by them, but the comment remained posted by [deleted].
This was all a year or 2 ago, so I don't remember the exact situation any more.
It's not just accounts that are bought and paid for. Every major social network is making money by shaping conversations around the interests of paying customers, political and otherwise.