Final answer:
The correct action when a certified person determines no refrigerant remains in a system is to seal the system and label it, complying with environmental regulations and proper maintenance protocols.
Step-by-step explanation:
When a certified person determines there's no refrigerant remaining in a system, they should seal the system and label it. This is an important step in ensuring that repairs or disposal of the equipment are done according to environmental regulations, typically outlined in the protocols for refrigerant handling (for example, the EPA's guidelines in the United States). The sealing process ensures that no contaminants enter the system and that it's clear the system has been evacuated. Labeling provides information about the system's condition for future technicians or handlers.