FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
As of FortiOS version 5.4 the Banned User List is viewed with a new CLI command.

The commands for v5.0 and v5.2 which are mentioned in KB article FD36211 for viewing and manipulating the list are no longer available.

On version 5.4 the command below may be used:
# diagnose user quarantine list

Other available commands for deleting/adding to the list, clearing the list, and obtaining statistics can be viewed by typing the following command with the question mark at the end:

# diagnose user quarantine ?
list     List user quarantine entries.
add      Add user quarantine entry.
delete   Delete user quarantine entry.
clear    Clear all user quarantine entries
stat     Stat

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Technical Note: Viewing Banned User List using the CLI in v5.2 - diagnose firewall ip_host