Don't want Dictionary, Thesaurus, etc., interrupting Pages
How do I stop Dictionary, Thesaurus, etc. from interrupting Pages while I’m trying to write?
Uncheck everything in Pages > Edit menu > Spelling and Grammar > sub-menu. Visit Spelling and Grammar > Show Spelling and Grammar, and deselect Check grammar. Turn off anything that bothers you in the Pages Preferences > Auto-Correction panel. Do so in the System Preferences > Keyboard > Text panel.
If you are using Grammarly on the Mac, don't. That should just about do it for things that want to alter what you type.