Not a UX designer, but I would try worsening the UX of B/I/U/size operations to make users more inclined to reach for the formatting style picker instead. I would also heavily optimize the process to edit styles, so changing the font size of a heading changes all headings of that level unless you specify otherwise.
(In fact, as I talk through this, I think the solution may just be to abandon the idea that attributes like bold and size are applied directly to text, and instead associate them exclusively with styles. Basically the migration from <b> to <strong> at the dawn of CSS, in WYSIWYG form.)
Ultimately you'd end up with a word processor that's harder to use but easier to use right. And no sales team will greenlight that.
But this entire discussion is predicated on a very prescriptivist viewpoint of the "right" way to use a word processor.
