When you want to delete a lot of nodes or users you can use the delete all module in drupal. You have some handy drush commands:
drush delete-all article Delect all article nodes.
drush delete-all all Delete nodes of all types.
drush delete-all --reset Delete nodes of all types, and reset node, revision and comment counters.
drush delete-all users Delete users.
--reset Reset counter for node, revision and comment tables.
--roles pick roles