gitlab-org--gitlab-foss/doc/administration/user_settings.md

Modifying global user settings

GitLab administrators can modify user settings for the entire GitLab instance.

Disallow users creating top-level groups

By default, new users can create top-level groups. To disable this, modify the appropriate configuration file.

For Omnibus installations, add the following to /etc/gitlab/gitlab.rb:

gitlab_rails['gitlab_default_can_create_group'] = false

For source installations, uncomment the following line in config/gitlab.yml:

# default_can_create_group: false  # default: true

Disallow users changing usernames

By default, new users can change their usernames. To disable this, modify the appropriate configuration file.

For Omnibus installations, add the following to /etc/gitlab/gitlab.rb:

gitlab_rails['gitlab_username_changing_enabled'] = false

For source installations, uncomment the following line in config/gitlab.yml:

# username_changing_enabled: false # default: true - User can change her username/namespace