The best practice for preventing a session from being derailed by an off-topic stakeholder is to use a "parking lot" technique. A parking lot is a list of topics or issues that are not relevant to the current session but may need further discussion or resolution later. The BA should add the stakeholder's concerns to a parking lot for further discussion, and acknowledge that they are important but out of scope for the current session. This way, the BA can keep the session focused on the onboarding process and avoid wasting time or losing engagement from other stakeholders. References:
https://trailhead.salesforce.com/en/content/learn/modules/business-analysis-process-mapping/plan-and-facilitate-