There is no plugin that on its own will prevent griefing all together. The best protection is regular maintenance, and a group of carefully chosen plugins.

Here is a list of a few that will help you. Keep in mind that most of them require a little reading.

#1 Permissions. To keep control of who can place, and destroy blocks in the first place.

#2 World Edit, and World Guard. In the rare case that anyone does manage to get griefed you can easily fix any problem.

#3 LWC or Lockette. These both do the same thing lock anything that can be opened. I recommend LWC since you do not have to place a sign every time you want to protect something.

#4 HawkEye. This lets you know who placed what block, and even gives you the option of rolling back their deed. Simpler versions like LogBlock, and BigBrother are available but do less things.

#5 SafeExplosions. Keeps Creepers, and TNT from messing with your stuff.

#6 OwnBlocksX. This one is the only one that might be a bit risky. It makes it so that any block placed by an individual can only be destroyed by him/her. You can see how this could be used for griefing but also could provide griefers with a tool. I Dont let my users know that I have this one so they dont get any ideas.

Make sure you configure these right before you use any of them, otherwise they are useless.

And the links to the aforementioned plugins:
Permissions: http://tinyurl.com/4y6wfzn
WorldEdit: http://tinyurl.com/cpclurh
WorldGuard: http://tinyurl.com/6g3tbt9
LWC: http://tinyurl.com/7f7jm7v
Lockette: http://tinyurl.com/77haywq
HawkEye: http://tinyurl.com/6vza4nx
SafeExplosions: http://tinyurl.com/6pw5pwv
OwnBlocksX: http://tinyurl.com/3t3abm7

If you need any help setting these up contact me and I would be glad to help.