Power BI developer can ensure report security by implementing Row-Level Security (RLS) to restrict data access based on users' roles. Define clear user groups and permissions within Power BI service. Data should be encrypted both in transit and at rest, and developers should work with the IT department to enforce secure access protocols such as multi-factor authentication (MFA). Use Power BI’s built-in auditing and monitoring tools to track access and changes. Additionally, avoid sharing sensitive information in reports unless necessary and provide thorough documentation on security practices to ensure the solution remains secure over time.