Here's the main problem with spam:
If people would stop clicking on the advertising links in the spam that's posted in email, on forums, in chats, in online games, and everywhere else spammers annoy the general public, then there would be no motivation for spammers to create spamming robots.
What is a spamming robot? It's not necessarily a single robot, but usually a collection of computer programs (or "scripts") which perform specialized tasks. On problem many website maintainers deal with are false user account registrations. This is a script whose only purpose for existance is to continually try to create accounts on a website. For example, within a day of this web site being online, a bot had found it and attempted to create 23 false accounts. One of my clients now recieves over 1,000 robot registrations a week, very annoying since they depend on live people for thier income and have to sort out all the spam bots (this is after the CAPTCHA blocks most of them.)
Speaking of CAPTCHA, this used to be a pretty good tool for filtering out the robots from the humans. Since we humans have incredible nural net brains specialized in pattern recognition, web designers created slightly distorted image of numbers and letters which are easy for humans to read but baffle the bots. Thus for years, CAPTCHA was the standard for keeping bots out. However, bots became better at pattern recognition, and CAPTCHA images became nearly impossible for even humans to read as a result.
AYAH tried a new approach of creating very simple flash games, again easy for humans and baffling to robots. Games were simple like, "Put the balls in the toybox" with a drag-and-drop interface. Again, this worked for a while until the bots created a new approach that was nearly unbeatable, human farms.
There are towns scattered around the world that make their entire revenue by spamming and scamming Americans. One town with a human farm will pay humans $.25 for every 100 CAPTCHA codes and AYAH games they solve. Yes, spamming is so profitable that they can support entire local economies. This means that there is now nearly no possible way to entirely prevent bots from creating accounts on your web sites.
Now that bots are going to invade your website, they still need to post their annoying and unwanted spam. Fortunately for us, that is still almostly completely automated. Unfortunately, that usually means web designers have to add CAPTCHA or AYAH to every post to prevent bots, but also annoys their clients to the point of cancelling accounts.
BOTCHA is a newer weapon in the war againts bots. Where CAPTCHA requires you to prove you're human, BOTCHA tricks bots into proving their not human. As an example, BOTCHA will add extra, hidden fields to a form. Humans can't see the form because the web browser won't display them, but bots don't use web browsers and see every field and try to fill them out. This is onle of the methods BOTCHA uses to trick bots. The best part is humans don't need to prove they are human.
It's not fool proof in that every spam and bot prevention method is susceptible to human farms, but it's a great start.
As I write this, "The Matrix" and "The Terminator" keep coming to mind. We used to depend on bots to help us, now bots are depending on our help, and needing less and less help every year.