There is a simple toggle button to turn Spellcheck on/off in Notepad settings.
Although it wouldn't have helped in this case, I do love the irony of a good typo in a post calling spellcheck and autocorrect "crap".
Powershell, Get-AppxPackage *Microsoft.WindowsNotepad* | Remove-AppxPackage and get on with your life. It will restore the legacy notepad.exe (as will, like, right-clicking the new Notepad in the start menu and selecting Uninstall) -- but yeah, I know, you didn't opt in to this, <insert rest of generic Microsoft-related HN complaints here>