how to prevent hacker or tighten security for my cobalt RAQ4?

What is the worst case you guys had seen before on your server? hacker hack your raq? How to prevent? what they use? How to check if there's a guy trying to hack or doing lame thing on my server?

I seached for 'tighten raq' and found a thread.

http://www.webhostingtalk.com/showthread.php?&threadid=8539&highlight=tighten+raq