Prohibit the use of portable storage devices when such devices have no identifiable owner.
Bold Coast Security Guidance
The clarification from CMMC suggests a policy and training driven approach to satisfy the prohibition of portable storage devices without a clear owner.
There are technical solutions available which would block all but known devices. Your organization should consider these solutions in your environment. For instance, if you utilize Microsoft Active Directory, you can disable the use of removable storage using Group Policy.
DRAFT NIST SP 800-171 R2
Requiring identifiable owners (e.g., individuals, organizations, or projects) for portable storage devices reduces the overall risk of using such technologies by allowing organizations to assign responsibility and accountability for addressing known vulnerabilities in the devices (e.g., insertion of malicious code).