To disable the app bar in SharePoint Online, you typically have to use SharePoint admin settings. However, Microsoft's capabilities and settings may change over time, so it's a good idea to check the most recent documentation and admin center settings for the latest options.

Here's a general outline of the steps you would follow through the SharePoint admin center:

1. Sign in to the SharePoint admin center: Go to the SharePoint admin center (https://admin.microsoft.com) and sign in with an account that has SharePoint admin privileges.
2. Configure the app bar settings:
3. Click on "Settings" in the left-hand navigation menu.
4. Select "Organization settings."
5. Look for options related to the app bar or global navigation, and configure them according to your preferences.
6. Save changes: After configuring the settings to disable the app bar, make sure to save your changes.

Please note that the specific steps and options might vary depending on the updates and changes made by Microsoft to SharePoint Online. Therefore, it's essential to refer to the latest Microsoft documentation or consult with your organization's SharePoint administrator for the most up-to-date instructions on managing the app bar.