As the issues that this tag focuses "office-onenote-client-itpro" on do not include iOS devices, to help you better, I would suggest you post a new thread on Microsoft Answers forum. Thanks for your understanding.
OneNote for iphone or iPad
The following content is based on my personal tests.
I download install the OneNote app on my iOS device, under the Settings, there is indeed no "Account" option.
But if your user wants to remove account, please try to switch to the account needs to delete, click the avatar icon in the upper left corner > Click the account > In the pop-up account list, click Sign Out, and then there would be a prompt like "Do you want to remove your account?".
Hope the information could be helpful.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.