How do I permanently disable Apple Intelligence?

It will probably get re-enabled with each update or upgrade. That's what's happening on iOS (iPhones, iPads) ... so you should anticipate it also happening with macOS.

You're just going to have to check each time & turn it off if necessary.

Let Apple know here:

Product Feedback - Apple

Matthew Lyon wrote:

I turned off Apple Intelligence when it was first available. With every system update, it is re-enabled. I do not want this feature enabled.

Disable it again.