how to disable users

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There is no such option, but you can turn user into "blocked", then he or she won't be able to do anything on the forum, except log in and send feedback to the administrators.

Effectively such user is locked out, so I don't really see a need to prevent them from logging in. What do you think?

There is a tab "moderation" in each user's profile that is available only to moderators and administrators, where you can change user's status.

To make first admin at this point you need to run a command line:

python manage.py add_admin <user_id>

But, I will make first user who register to become an administrator automatically soon.