I would like to add to this. Everyone including server admins should be copying and pasting this message on their forums to help spread the word.
The next thing to do is inform your users on how important it is not to download stupid silly hacks from the internet. Almost all the malware keyloggers and crap come from this.
In fact today maybe minutes ago someone on my server seam to have been infected by some kind of malware. It seams to insert it's self into chat conversations. I thought it was packet releated but then it would not make sense why it did not start till the person hit t button.. Notice how it's his where should be this.
Check a look at this image:
Now who knows if this is a Minecraft one but the best part of it was this links to a exe file that clearly is keylogger/malware.
SO EVEN MINECRAFT players are at risk so watch CHAT. Always tell players not to download files from other players. And if you download a texture pack make sure it's just images and not class files.
It's best to not download any client mods that are not from respectable authors who have been doing it for moths and NEVER download special modified mods from say other people in the threads and such. They might just be a trap waiting for you!
Never give out your password to anyone ever. No clanilla mods or admins will ever need your pass so dont be fooled by anyone impersonating a staff member. report all attacks on clanilla to clitcomander, be it griefing or other wise.