Suppose Fail2Ban is already installed, if not, then see my article – Installing and Configuring Fail2ban.
In Fail2Ban by default, there are already filters for ProFTPd and it knows that the log file is located at /var/log/proftpd/proftpd.log, so it’s enough to create the file:
sudo nano /etc/fail2ban/jail.d/proftpd.local
And enter the data below, thereby activating the check of the log file /var/log/proftpd/proftpd.log:
[proftpd] enabled = true bantime = 86400
Restart Fail2Ban to apply the changes:
sudo service fail2ban restart
You can check the status:
sudo fail2ban-client status proftpd