How can you stop griefers? By making their efforts useless. If they delete a whole map full of great buildings, and you can restore it in a few seconds, then they're simply wasting their time. If every server can do that, then their motivation to grief is incredibly small.
So, I'm going to show you how to do that, by having backups for every minute the server has been running! And not only that, but I'll let you do it without taking up any hard drive space! All you need is an active admin who'll be around to revert the backups and restart the server, everything else is automatic.
Ok, to do these backups, we're going to use Dropbox.
We're going to use the Dropbox versioning to allow us to keep versions of the server_level.dat file. Versions don't count toward your space, and you get 2GB for free, which is plenty. The Minecraft server saves server_level.dat every minute, so Dropbox will save a version of it every minute.
3. Move your server to My Dropbox
This is the folder that Dropbox synchronizes.
4. You're done!
Make sure everything has green checkboxes by them, which means Dropbox has them fully synchronized, and start your server. Now, every minute when the server saves the level, Dropbox will upload the file. Note, the server.log file will never upload while the server is running, because Minecraft is using it.
Restoring Your Backups
Oh no! Overnight, someone came through your server and deleted practically everything! Time to restore!
However, before you restore, you should think about it. Restoring will remove everything in your server that has been built since the time of the backup. It will also require you to restart the server, kicking everyone off. If you're sure you want to restore, then read on: