In your campaign's dashboard after picking and confirming a winner, you have the option to remove him/her from your campaign.
To do that, simply tick the box on the left to mark the winner and then click the "Remove" button.
This option might be needed in case someone is not eligible for the prize or in case you, as the campaign admin, decide to include the winners in future giveaways (since every winner is excluded from the participants pool next time you decide to pick a winner).
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