PLUGIN? ‘Bulk Delete’
What a wonderful time saver this plugin is. There was over 1400 user accounts (spammer bot created accounts) and as I started to delete them, it was going take forever.
A quick search in the plugin-search box turned up this little gem
And in an attempt mitigate further automated account creation, the ‘Restrict Usernames’ plugin (with a criteria that is difficult) has been installed to stop this activity.