SimonG has a post on blog comment spam:

So, there are lots of ways to stymie the spambots. Why, then, am I about to tell you my way? Wouldn’t it be better if I encouraged you to go off and come up with a technique of your own? Surely if everyone used a different method, it would be harder for the spammers to get round them all? That is true, but I’m not sure that’s necessarily a good thing. I want the spammers to get round my traps. When they do, I’ll add some more. It’s an arms race, and it’s in the interest of those of us who despise spam that the race moved forward as quickly as possible, because we’re guaranteed to win it. We have two big advantages over the spammers. 1) It’s very hard to write a program that can pass a Turing test, but very easy to make a Turing test; and 2) no matter how smart they get, it’s simply impossible to make spam comments indistinguishable from real comments because, when it comes right down to it, there is a difference. If there wasn’t, they wouldn’t be spam. It might be that when the difference becomes subtle enough, only advanced AI techniques are able to detect it, and perhaps if the arms race goes too quickly we’ll reach that point before such techniques exist, but I don’t think that will be a problem. My message to the spammers, then, is a simple one: Bring! It! On!

He then goes on to describe what he's been doing to defeat spammers, though he warns readers:

Let’s get down to the details. For those of you not of a technical bent, this would be a good time to go and put the kettle on. Alternatively, here are some pictures of kittens. In fact, unless you’re a codey type person with an unhealthy interest in HTML, I seriously advise you not to read on. Go and look at the kittens instead.