Get evidence and report him to the Sr. Staff. PM them, and if they don't respond within a week or so refresh the PM - a Moderator threatening to abuse their powers is not an issue to be taken lightly. Screenshots, chat logs, everything.
He doesn't like the default font. It makes him more comfortable using one that he likes rather than the default. I guess there's nothing wrong with it, but it also doesn't serve a particularly useful purpose. However, from my point of view, it makes his posts more appealing, and stand out a bit more. Even if they're only a few lines, it shows that he pays attention to everything he posts, and took the time to put in effort.