Delete fake users and check security

[matamadio]

Currently, the geonode has some 28000 users, about 27700 of which are registered by bots with fake email.
This creates issues in the admin interface, e.g. every time there is a "users" dropdown list, it parse all the 29k records and the browser crashes.
Also, the geonode goes 502 for a while.

We need to keep only the urser that satisfy one of these conditions:

1. staff users
2. non-empty "name" and "surname" fields
3. email "worldbank.org"

The django admin interface allows to delete about 20 users per time (edit: it doesnt matter the number, but there are some users that it fails to delete, aborting the operation, example), which is not optimal for this task. Is there a better way to remove those users from db?
Can we put implement anti-spam to block further fakes? if not already in the 2.10 update.