Who is hammering FTP on our server?

sudo grep "no such user"  /var/log/syslog | cut -d[ -f 3 | cut -d] -f 1 | sort | uniq -c | less
What firewall rules do we have in place?

sudo iptables --numeric  --line-numbers -L INPUT | less
How do we add to the rules immediately?

sudo iptables --insert INPUT 3 -s xxx.xxx.xxx.xxx -j DROP