You can review the assignments for your challenge in several different categories.
If a work is posted that does not meet your challenge standards, you can remove it from the collection under the Manage Items page and the work will be removed from the listing, at which point you will be able to come back to the assignments page and mark the writer as having defaulted.
NOTE: if your collection is unrevealed or anonymous, removing the work from the collection will cause it to be revealed!