Unfortunately Brute force attacks are a common phenomenon on the internet. See http://en.wikipedia.org/wiki/Brute-force_attack for some background information.
When you're using strong passwords you don't have a reason to be very worried about this. If you want to do something about it you can run your SSH daemon on an other port or install a program like fail to ban. You can edit the ssh port in /etc/ssh/sshd_config Don't forget to restart sshd ;)
It's possible to disable the brute force attack notifications in DirectAdmin via:
Extra Features --> Administrator settings --> Parse service logs for brute force attacks --> No