From the #unleashyourgeek and Unleash Your Geek technical handbook
You first need to open phpMyAdmin and select your Drupal database.
Once you have selected your database you need to go in and select the SQL command button and enter the following command:
SELECT node.* FROM node
LEFT JOIN users ON node.uid=users.uid
WHERE users.uid IS NULL;
This command tells your database to delete all the nodes where the user information is null, or nothing. This occurs when Drupal deletes the user account from the database. Here is a screenshot of the command. Hit GO after you cut and paste it.
It may take a couple of minutes for this command to work, so don’t panic. The more nodes you have in your Drupal database the longer it’s going to take. When it comes back you’ll see the following. You want to select all the results and hit the red X for delete.
OK, you’re finished. When everything goes according to plan you’ll see the following.
Open your site and make sure to clear your browser’s cache. You should now see the N/A errors are gone