By default, the app allows user's to login via Cloudron LDAP and via email. Once you have made one or more Cloudron user's as Redash admins, the email login can be disabled. For this, use the File manager and add/edit a file /app/data/env.sh and add the following line:
export REDASH_PASSWORD_LOGIN_ENABLED=false

Restart the app for changes to take effect.