Configure the policy value for Computer Configuration >> Administrative Templates >> MSS (Legacy) >> MSS: (DisableIPSourceRouting) IP source routing protection level (protects against packet spoofing) to "Enabled" with "Highest protection, source routing is completely disabled" selected.
This policy setting requires the installation of the MSS-Legacy custom templates included with the STIG package. "MSS-Legacy.admx" and "MSS-Legacy.adml" must be copied to the \Windows\PolicyDefinitions and \Windows\PolicyDefinitions\en-US directories respectively.