First go into the wp_users table and delete any users you need to.
Then go into the wp_usermeta table and delete any meta entries that correspond do the IDs of those deleted users.
Then you need to alter the wp_users table to reset its auto increment value. So you can do that in phpMyAdmin, or use SQL like this:
ALTER TABLE `wp_users` AUTO_INCREMENT={ID you want to be used next}So if you want the next created user to have the id number 4 it would look like this:
ALTER TABLE `wp_users` AUTO_INCREMENT=4