Suppress the address first
Add the recipient to a durable suppression list so routine imports or campaign retries cannot silently re-add them. Keep only the information needed to honor the suppression, and ensure every sending path consults the same list.
Do not contact the person to ask them to reverse the complaint. If they later choose to subscribe again, use a clear, affirmative sign-up flow that records when and how consent was given.
Investigate the pattern
Review the subject, sender identity, content, audience selection, and sign-up source for the reported campaign. Compare complaint timing across segments; a sudden concentration can reveal stale consent, confusing expectations, or a broken unsubscribe flow.
Document the corrective action and monitor the next carefully targeted send. If complaints persist, pause the affected source or campaign and verify authentication, list hygiene, and preference handling before resuming.