Hello,
Fail2ban automaticly blocks an IP address for some time when someone tries to brute-force your Pi.
Info: http://raspberrypihelp.net/tutorials/48-raspberry-pi-ssh-security-fail2ban
Just a matter of apt-get install fail2ban.
I like it already! 
Great suggestion, I’ll look into adding this as a default installed package, for all DietPi systems.
https://github.com/Fourdee/DietPi/issues/90