How to Remove OneCalendar from OneNote Ribbon
When you install Onetastic, it comes with OneCalendar and will add it to OneNote ribbon on Home tab. If you don't want to have it there, you can remove it by customizing your ribbon. To do so, right click on anywhere on the ribbon and choose Customize Ribbon.
From the Options dialog that opens, choose the Tools group on the right that contains the Launch OneCalendar button and choose Remove. This will remove the group from the ribbon. After that click OK.
You can use the same method for removing other Onetastic buttons or re-arranging them the way you like in the ribbon.
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