Delete a variable.
drush variable-delete
name : The name of a variable or the first few letters of its name.
--yes : Skip confirmation if only one variable name matches.
--exact : Only delete the one variable that exactly matches the specified name.
Delete the user_pictures variable.
drush vdel user_pictures
Choose from a list of variables beginning with "u" to delete.
drush vdel u
Bring the site back online, skipping confirmation. Variable is rewritten to site_offline for Drupal 6.
drush vdel -y --exact maintenance_mode