Unfortunately, this isn't going to be easy. There's a reason the C in
CSS stands for "Cascading"

Font attributes specified at the lowest level like that will be
implemented in one of three ways: (a) font tags, (b) a style attribute on
the tags surrounding the content, or (c) a class attribute. Only (c) offers
any hope of change, if you can figure out what the class is and override the
name. Not likely, though.

What if you use the other button (Source, I believe) instead of the one
that lets you enter rich text? Pasting into *that* text box should ensure
that no formatting is perpetuated.