It has 100% Religious Zeal preventing me from converting it.
Zeal only lasts a little while, in the meantime force convert in war will change someone's capital to your religion, that overcomes Zeal.
As France, trying to prevent the BI is a PITA. You could:
- Release vassals and feed them provinces so that you no longer have a direct border with Burgundy.
- Use wars to break Burgundy's PUs and take Burgundy's provinces that are not in the French region.
- Stay perpetually at war with the Holy Roman Emperor until 1500.
- Dismantle the Empire.
It's not that hard as France to get all the territory, become Emperor. It may involve fighting Austria to weaken them such that they lose the title. Release Styria gets you zero AE, but massively weakens Austria to the point you can probably steal the title pretty quickly, even if it involves vassalizing the electors, totally worth it.