Enter OSE Anti-hacker suite for Joomla. These people have developed a great product which (I'm pretty sure) does what it says it will do - I've already stopped 3 attacks in the first 24 hours of operation.
It's the only commercial extension I have bought thus far and the AU$85 purchase price was worth every cent. Not only that but the support has been incredible - super fast, professional and reliable. They're obviously setting about building a quality reputation and deserve to be commended.
If you've had major hacker problems you'll know what I mean when I say that it's fantastic to have a team like this on your side. I can't recommend them highly enough - it's very cheap peace of mind.