A possible solution could be for users to have an accuracy rating (perhaps short-term and long-term), and for the Turk API to take an optional minimum accuracy

A possible solution could be for users to have an accuracy rating (perhaps short-term and long-term), and for the Turk API to take an optional minimum accuracy rating for tasks to be visible to a given user. (For all I know this is already in place; I'm not a Turk user.)