Failed login attempts are logged in /var/log/ajenti/ajenti.log. A basic filter for Fail2ban is available here : https://raw.githubusercontent.com/ajenti/ajenti/master/scripts/ajenti.conf
You can enable it by copying it in /etc/fail2ban/filter.d/ajenti.conf and with the following lines in /etc/fail2ban/jail.d/ajenti :
[ajenti] enabled = true port = 8000 bantime = 120 maxretry = 3 findtime = 60 logpath = /var/log/ajenti/ajenti.log filter = ajenti
This is only an example : after 3 failed attempts ( maxretry ) the last 60 seconds ( findtime ), the found ip will be banned 2 minutes ( bantime ). You can naturally set other values related to your configuration.