AFAIK, mIRC's font linking is entirely done by itself. If you don't like that specific font, the easiest fix is to uninstall it and then mIRC can't link to that font. mIRC's displays are not your typical windows where you type and the font appears. You can see this simply by trying to highlight text... it doesn't work like other apps. I'm sure that relates to why mIRC does its own font linking. I doubt that is going to change any time soon, though that's just my guess.
