How do I stop the AI using certain words in my emails?

Open the Template, then Words, and add the word or claim under Never say, in any section. The AI must never use it in any section it writes. For example: guarantee.

For one section only

Each section also has its own never-say list, under its own rules, for words that only that section must avoid. The opening line has one too, and your never-say words for every section count there as well.

The subject and the opening line follow it too

If your never-say words rule out the usual wording, the subject and the opening line are written another way. For example, if you ban Congratulations, a new company gets a plain subject instead.

Words in your own sections

Sections set to My exact words go out exactly as you wrote them, so check those yourself.