A player's registration can be deleted from their Registration Edit profile. You can access a player's registration edit by either bringing up the registration edit window directly from the admin panel (Registration->Registration Edit) and then entering the player's registration number, or you can go to the registration list and click the player's name.

In the registration edit window, you will find at the top-right a field labeled "Reg. Status". Set this to "Withdrawn", and the player will be set to a withdrawn status. When a player is in this status, they are not considered to be active in the league for that season, and are therefore ineligible for being drafted to a team.