Someone is trying to hack into my FTP, he/she is trying for a few hours now with diferent usernames.

My daily log file is 8 MBytes of login tries from the same IP.

How can I block that IP or the whole network ?

PS
I found this website to locate by IP: http://www.ip2location.com/free.asp

And I got this:
IP Address Country (Short) Country (Full) Flag Region City ISP Map
124.0.210.117 KR KOREA, REPUBLIC OF - - SK NETWORKS CO

I won't mind if I block the whole network, ie, block "124.*"

But how ?