To prevent the prioritization meeting from turning into a brainstorming session, the product owner should ensure that user stories are well-defined before they are added to the backlog. By reviewing the items with the product owner's teammates (or stakeholders) beforehand, the product owner can clarify the details of the stories and ensure they are properly defined. This allows the team to focus on prioritization during the meeting, rather than spending time clarifying vague requests. This proactive approach improves the efficiency and effectiveness of the prioritization process.