Hello tugi,
Thank you for posting in Q&A forum.
Based on "But the user policy configuration doesnt work at all.", for the user policy, you should apply the policy to user account object.
Put the user account objects into an OU.
Link the GPO to OU with user account objects.
Edit User Configuration\Policies\Administrative Templates\System\Removable Storage Access.
Run gpupdate force on client with the specific user account logon in the OU above.
For the computer policy, you should apply the policy to computer account object.
Put the computer account objects into an OU.
Link the GPO to OU with computer account objects.
Edit Computer Configuration\Policies\Administrative Templates\System\Removable Storage Access.
Run gpupdate force on client with any account logon.
Reference:
https://www.prajwaldesai.com/how-to-disable-usb-devices-using-group-policy/
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou