[CentOS] Defending againts simultanious attacks

Hi,

i have one centos 4.3 box, exposed to the internet.
since several weeks ago, i found numerous attemps to connect through
SSH, but failed.

they tried with many username, including root.
it's comes from different IP. some of them are foreign website.

How do i make my centos become smarter in handling this kind of attacks.

eventhough i've disable all the user accounts, left only the admin
accounts. making the password so hard, longer and combining alphabet,
numbers and characters... yet i dont want the attackers keep on
trying.

any suggestions?

thanks in advance.