You can very easily clear a player out of the box that player owns, or even remove the player entirely from the Contest altogether, including any boxes that player owns.
This activity is done from the Players Settings Page, from within your Contest Settings.
Removing Players from the Squares View
Near the top of the Players page, you'll find a simple view of your Contest Squares.
Simply click on any of the boxes within the Contest Grid and you'll be given options, including both the option to remove the player from that single box, or to remove the player entirely from your Contest.
Removing Players from the Listing View
Below the Squares View, you'll find a directory listing of each owned box in your Contest.
Simply click on the Edit Button next to any player's name and you'll find the same options described above, including the option to remove the player from the specific box or entirely from the Contest.