Try the basics first, i.e. hiding of the posts from new accounts, IP and cookie blacklisting. See if it works. Then move on to more advanced stuff, e.g. browse

Try the basics first, i.e. hiding of the posts from new accounts, IP and cookie blacklisting. See if it works. Then move on to more advanced stuff, e.g. browser fingerprinting and behavioral pattern matching. If it's automated, there'll be a pattern. But the basic stuff can go a very long way. As a bonus, you can put all new accounts into a single group, so that they would see each other posts, but without making these posts visible to the approved accounts.