The auto update feature in Kodi refers to the update of add-ons from their repos.
The updating of Kodi itself would need to be prevented at the operating system level. If you’re using Android, then it was probably Google Play Store that auto-updated your Kodi. If you’re using Windows, then the Windows Store would have auto-updated Kodi.
But, while on the topic, what problems are you having with Kodi 18? It’s working great for me and most everyone else.