You can delete dashboards (and other pages, including wiki pages and presentations) that you have personally created or that you are the 'owner' of, meaning that someone made you the owner of an existing dashboard.
To delete a dashboard, from the left navigation panel, go to My Resources > Pages > My pages.
Hover over any dashboard you want to delete. You will see a trash bin icon appear in the top right corner of the dashboard tile. Click the icon and confirm you want to permanently delete the dashboard.