How to set Drush sql-sync not to clone users between @dev and @staging?
The drush sql-sync has an option --skip-tables-key that allows you drushrc.php to skip tables during the synchronization.

In your drushrc.php you add this line. 

$options['skip-tables']['staging'] = array('users', 'users_roles', 'roles', 'permission');


Your drush command line should look like the following.

drush sql-sync --skip-tables-key=staging @dev @staging


You've to be careful, as you've to skip all tables that had informations related to your users, including the tables used from third modules.