I think that's a great example of where "tone" is absolutely not sufficient, especially in the modern world where many readers aren't going to share your first language and won't absorb the nuance.

If there's a situation where your disagreement is absolute, you need to say that factually, ideally with a recipe for how to resolve that included:

"I can't approve this scheme, please do X instead."

"This API isn't OK to use here, you have to..."

"Under no circumstances will I ever approve of a feature that does this."

You can phrase those as nicely as you want, but it's imperative that your disagreement be spelled out explicitly.