To manually block an IP address accessing ProFTPd you can use the following in the ProFTPd config file:
<Limit LOGIN>
Order deny,allow
Deny from 111.222.111.222
Allow from all
</Limit>
</Global>
Of course it would be better to use IPtables
<Limit LOGIN>
Order deny,allow
Deny from 111.222.111.222
Allow from all
</Limit>
</Global>
Of course it would be better to use IPtables
iptables -A INPUT -s 111.222.111.222 -j DROP
To allow the IP address again:
iptables -D INPUT -s 111.222.111.222 -j DROP
Where -A is append and -D is delete
Even better would be to automate all this and use fail2ban 