Apologies for getting late back to you.
To answer your query, yes you can delete leaderboard as well as individual entries from that particular leaderboard directly through the dashboard. Please follow below steps and let me know if you need any help from my side:
Delete Game :Select App42 Cloud API -> Gaming -> Games
Click on Delete Game button given on the game icon and submit it will delete your leaderboard from the backend
Delete Individual Entries from leaderboard: Select App42 Cloud API -> Gaming -> Leaderboard
Select your leaderboard from which you want to delete the particular user score
Click on Manage -> Delete Score to delete the user score from that leaderboard