Doxxing is not ok, the threat to do so is not ok either but a little grayer. Imgflip can give a warning, delete comments, disapprove or delete images and/or delete account(s).
I'm happy to take a look at a situation, but I'm not on a lot lately so there might be a delay if I receive links or specific info via a comment or memechat. I would say any sitewide moderator is ok with taking a look at something like that or other content/conduct against the Terms of Use.
Comment flags or image flags could work if they are reviewed and action is taken by a sitewide moderator, a better option could be contacting global moderators directly with details via
imgflip.com/request_removal