[OneNote has taken over my paint.net]

Last week I found I had the EXACT same problem you discovered.

I've never (knowingly) downloaded Onenote, or activated it, or made any changes to Paint.net

I just started seeing the print function of Paint.net getting hijacked by Onenote, a program I still have no idea what it's used for.

 

Basically, you have either (1) uninstall OneNote or (2) deactivate it from Microsoft Office.

For me (Windows 7), this took the following:

1) Click on [Start] Button

2) Select [Control Panel]

3) Click on 'Uninstall a Program'

     A list of all programs on your PC appears

4) Right-click on your version of MS-Office

5) Select 'Change'

6) Select 'Add or Remove Features'

7) Click the [Continue] Button]

    A menu of all possible features of MS-Office appears

8.) Click on pull-down button [V] to the right of OneNote -- NOT the expand button [X] to its left

9) Click on [Not Available]

10) Click the [Continue] Button

    MS-Office will re-configure itself, which may well take several minutes.

11) Click the [Close] Button.

12) Bring up a program in MS-Office.

 

https://www.techwalla.com/articles/how-to-disable-microsoft-onenote

is where I learned this.