User suspected of manipulating votes, but we don't have the tools to do anything about it.

Hello! I'm posting this from an alternate account to not make it immediately obvious to the user.

Context: I moderate a small roleplay community, where users take on the role of a character inhabiting our virtual world and write stories together. Using more than one account to play multiple characters is fairly common and we don't have a problem with that. However, we have one user with 20+ known accounts - this is definately excessive, and to be honest I'm not sure how this hasn't triggered any filters on reddit's side.

Now, we've noticed a trend: if this user doesn't like you, your post gets immediate and consistent downvotes. Not just one, we wouldn't be making a fuss about that. It's fairly common to receive 3-4 downvotes (as we're a small community this is very noticeable and multiple users said they're annoyed by it), sometimes characters this person particularly dislikes just get downvoted into oblivion. I suspect they're using a script for this, because posts commonly get downvoted seconds after posting, even in the middle of the night. We caught them doing it in the past by blocking all accounts (mysterious downvotes were gone) and then unblocking (mysterious downvotes were back immediately) but now they seem to be using an unknown alt account to vote.

Now, if our suspicions are true, downvoting using scripts and multiple accounts would be pretty clear vote manipulation. But this person seems to be avoiding the automated filters and we haven't really been able to do anything except tell affected users "yeah it's probably that one guy". Is there anything we can do here, or even any tools to figure out who keeps downvoting our players?