You can only delete a registrant from a special event if there aren't any payments associated with that registration.
  1. Go to Events > Event Search
  2. Search for and open the event record
  3. Select the Registrations tab and click the drop-down next to the registrant's name there
  4. Click Delete
NOTE: If you don't see a Delete button, you will need to have the 'Delete Permissions' system role