If you are sharing a dashboard with person or list and would like to remove access you will want to start in the specific dashboard. From here, you will want to click the Share tab in the top right of your dashboard as shown below:
From here, a window displaying all people you have shared access with over email. To remove them, click the X to the right of their name
This will remove their access to view your dashboard. Please let us know if you have any additional questions on dashboard viewing!