Virtual blight. This 21 minute video is from Matt Cutts about web spam. It has some nice easy to follow examples about the types of blog spam, and how to deal with it.
There are some simple tips such as do not allow users to insert code. And use NoFollows on outbound links especially from commenters who you dont know and trust.
He suggests that if you are able to perform a security audit. And if you arent able to do this yourself ensure that you patch your software frequently. You can ask your web host about their security too.
One tip was to use Google alerts for your site, definately on words that should never ever be on your site e.g. viagra
You can also use your webmaster console and see search terms and or words on your site. If strange words appear you might have been hacked. If you know about it quickly you can do something about it and not look like a victim for others to join in !
Just watch the video. He explains it very well.