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 as shown below:
From here, a window displaying all people with access to your dashboard will be displayed like the window below:
Next, you will want to click the trash can icon to the right of the specific person. Note: If you would only like to remove access for a person from a list you will need to remove them from the respective list in the "Lists" tab first.