What about the ones who are actually making good posts in a thread where merits are off? I don't think it's reasonable to disable merits for everyone in a thread only because a few users might try to get merits by any means. Besides, the ones who are giving them merits must think they deserve it, and merits are not moderated, as far as I know, but that's not the actual point here. The point is, it's an injustice to those who actually make good posts and deserve getting merits.

A more viable and reasonable solution, imo, would be to remove merits from a user's profile/account if they have got them in a self-moderated thread, and their post got removed by the creator of the thread. I suggested something similar before, but I said it should be done even for deleted posts from moderators, but since we are discussing this issue, I think it's at least a good solution for this problem.
Now, if something like that is applied, users who don't want merit abusers to come in their thread, and try to obtain merits, and if they do and get merits for posts that they believe don't deserve it, they can remove those posts, and the merits they've received will also get removed from their profile.