Honestly, this is one of those things that never fully goes away - you just get better at containing the damage. Formatting drift from sales and SME drafts is basically a law of nature at this point. Here's what's actually worked for me:
Locked templates where the writer has almost zero formatting decisions to make. If the heading styles, font sizes, spacing and colour palette are baked into the master slide and the only thing they can do is type into designated boxes, you cut the nonsense down massively. Google Slides and PowerPoint both let you enforce this with master slide restrictions. Word and Docs are more of a pain but enforced styles help.
An one-page style card stuck to the template. Not the forty-page brand bible nobody reads. Just the stuff a non-designer actually needs: which fonts, hex codes, bullet point rules, and a few "don't do this" examples. People will read one page. The full guide? Gathering dust.
A single review checkpoint before anything reaches the client. Not a full revision - ten minutes from one person who owns the brand standards. Making it someone's named responsibility rather than a vague "someone should check this" expectation is the difference between it actually happening and falling through every single time.
Where it still falls apart: urgency. When someone needs to fire something off fast and the template isn't right there or the reviewer isn't available, the draft goes out raw. The only fix is making the templates genuinely quick to use, not buried in a folder no one remembers the path to.