It's sometimes hard to stop people talking about something! But since the council has no power to adjudicate on a complaint, there is little point to the discussion. It cannot be an agenda item - again because the council cannot decide. It is open to other councillors to put a motion to move to the next item of business. Or the chairman could ask members to refrain from discussing a matter which cannot be decided by the council.
Sometimes complaints are justified, other times citizens can be pretty unreasonable. It's just part of life as a councillor. Of course, if the council could lawfully do something that would pacify the complainant, whether they are right or not, that might be a desirable outcome for everyone.