How to unlink my Documents folder from my OneDrive account?

According to the Microsoft Support article linked by vembutech, you can use the following workaround.

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Important

Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.

To work around this issue, follow these steps:

1. Right-click the Windows logo at the lower-left corner of the screen, and then click Run.

2. Type regedit.exe and press Enter. If User Account Control window pops up, click Yes.

3. In Registry Editor, browse to the following path: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

4. Refer to the following table to find the registry key for the folder that encounters this issue, and change it to the default value.

+-----------+----------------------------------------+-------------------------+
|  Folder   |              Registry key              |      Default value      |
+-----------+----------------------------------------+-------------------------+
| Downloads | {374DE290-123F-4565-9164-39C4925E467B} | %USERPROFILE%\Downloads |
| Desktop   | Desktop                                | %USERPROFILE%\Desktop   |
| Favorites | Favorites                              | %USERPROFILE%\Favorites |
| Music     | My Music                               | %USERPROFILE%\Music     |
| Pictures  | My Pictures                            | %USERPROFILE%\Pictures  |
| Videos    | My Video                               | %USERPROFILE%\Videos    |
| Documents | Personal                               | %USERPROFILE%\Documents |
+-----------+----------------------------------------+-------------------------+

5. Restart the Explorer.exe process to make the changes take effect. To do this, you can use either of the following steps:

• Restart the process in Task Manager.
• Sign out, and then sign in.
• Restart the computer.

You can now go to settings and click on Backup> Manage Backup and Stop backing up the Desktop, Pictures and Documents.

No need for editing the registry. It takes about a minute to reverse the OneDrive "protect your important folders" steps.

Select the OneDrive (white or blue cloud) icon in the Windows notification area, and then in the activity center, select More > Settings > Auto Save > Update folders. Now deselect the folder(s) you want to make local and then choose to stop protection (no more cloud cloning/syncing).

https://support.office.com/en-us/article/sync-your-documents-pictures-and-desktop-folders-with-onedrive-d61a7930-a6fb-4b95-b28a-6552e77c3057

Every folder you deselect will be created for you on your local disk in the default location (and more*, read on). The process will provide shortcuts in these folders and their subfolders* called "Where are my files" which is a link to the corresponding OneDrive cloud location. *It also means your new folders may have a bunch of empty subfolders (save for the aforementioned shortcut). Your new folder's structure is a fileless clone of the corresponding previous folder's structure which still exists on OneDrive. Each (sub)folder only contains the cloud shortcut and any subfolders, but nothing more. In the newly created local folder, prune away any subfolders you don't want and delete any of the shortcuts you have no need for.