Allow presave hook to prevent saving of submission data

Hi,

webform_submission_insert has the following loop to implement the webform api:

    foreach (module_implements('webform_submission_presave') as $module) {
      $function = $module . '_webform_submission_presave';
      $function($node, $submission);
    }

The problem is, subsequent to the loop there are no checks made to ensure the submission is still valid. What I'm angling for here is a way to allow the presave hook to invalidate the save operation by removing the entry such that
empty($submission) === TRUE

(or some similar condition that achieves the same effect. Is this possible? Do you have a preferred approach?