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?