You can accomplish this by converting the users from on-premises users to cloud-only accounts.
Guidance from support has been to convert the user by following these steps:
- Move the Active Directory object to non-synchronized OU, and have AAD delete it from AAD
- Run the delta sync
- Run the delta sync again
- Restore the user from the cloud
- Restore the user from the local AD
- Run delta sync. The restored user will be a pure cloud object. Try to update its email address or department from cloud.
Related threads, for reference:
Stop syncing on-prem OU without deleting AAD/O365 users?
Changing an "On-Premise" Synced user to CLOUD ONLY