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You can remove your personal dashboards. Personal means that you are the owner of the dashboard. You can become owner of a dashboard by a) creating it from scratch or b) if someone else will make you the owner of an existing dashboard.
To remove a personal dashboard go to your profile by clicking your name in the top right corner and then clicking My profile.
Then switch to Visualizations tab and go to Personal. Hover an arrow over the dashboard you want to remove and click X icon. After confirming the deletion your dashboard will be removed.