Hey @softboy99, 
Check this short thread, it shows exactly where to click:
For spellcheck you set it to off via initialization config:
https://api.onlyoffice.com/docs/docs-api/usage-api/config/editor/customization/customization-standard-branding/#featuresspellcheck
That should stop all the red underlines instantly.
If anything else pops up, just let me know! 