Prevent user from syncing their personal OneDrive Accounts through GPO.

Use the Group Policy Editor

1. Type gpedit in the search bar. Launch the Local Group Policy Editor.
2. Navigate to User Configuration → Administrative Templates → OneDrive.
3. Enable the following option: Prevent Users from synchronizing personal OneDrive account.
4. Save the changes.

OneDrive: Prevent Users From Syncing Personal Accounts – Technipages