Removing an email account from Newton ends Newton's access to it. For Gmail, that's all you need to do. For Outlook and Microsoft 365, also remove Newton in your Microsoft account, because Microsoft doesn't let apps cancel their own access.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to the Accounts tab and select the account.
Click the trash icon.
In Remove account?, click Remove.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.