Any and everybody can and should be involved in writing stories: Product Owner(s), Scrum Master(s), Team, Stakeholders. The person writing the story should have a discussion with the PO to explain why that story is important.
The technique that limits the amount of churn is that the Product Owner should be the person with “Content Authority” over the final say in terms of priority.
- Why are you holding round table discussions on each story?
- How many stories are you finishing per sprint (if you are doing sprints)?
- Do you have a product owner (or person with the final say on prioritization)?