How to edit/extend/remove pager in the admin back-end?
Views Bulk Operations was written to handle such situations:


  This module augments Views by allowing bulk operations to be executed on the displayed rows. It does so by showing a checkbox in front of each node, and adding a select box containing operations that can be applied. Drupal Core or Rules actions can be used.


You can build a View to filter users by whatever criteria makes sense, and use VBO to delete them all (not just those on the current page) in one go.