How to restrict access to the discussion
Dashboard Assistant stores the discussions in a Jira project created for each dashboard.
By default, that Jira project acces is Open

An easy way to access the Project access page directly is to toggle the Restriction setting when configuring the Dashboard Access Review Gadget.

It will display the link to the project roles page.
From there, it is possible to change the project access setting

You may change it to Private mode and add the list of users who has access to the dashboard.
For more details on managing project access rights, you can refer to the Jira Cloud user guide.