How to permanently disable Grammarly on iPhone 14 Pro Max?

Delete the Grammarly keyboard from your iPhone.

Add or change keyboards on iPhone - Apple Support

First to disable the extention go to Settings > Safari > Extensions > Turn off Grammarly. Then on your App Pages hold on the Grammarly app and tap Delete app to uninstall.

If you are referring to iOS's Keyboard and Typing tools, go to Settings > General > Keyboard > Turn off any or all switches you don't want, such as Auto-Capitalization and Auto-Correct.