Once a player is on a released leaderboard you can't remove them entirely (otherwise their name would appear as a blank on previous releases) but you can exclude them from future releases.
To exclude a player go to the Players page, find the player and click the Edit button. Under Playing Status change from Included" to "Excluded"
You can also exclude players in bulk via the Players tab:
- Click the checkbox next the players you wish to exclude
- Click on the "With Selected" tab at the top of the checkbox column and select "Exclude"