Within Exclaimer Cloud Signatures for Office 365, folders can be used to organise signatures, and to restrict access to specific users. Admin users can access all signatures in all folders, and they can also restrict editor access to folders.
You are an admin user, and wish to prevent another admin user from accessing signatures in a given folder.
It is not possible to restrict folder access for admin users - only editors. In this situation, you would need to demote the user account from admin to editor.