Akismet

From the Akismet home page:

Akismet is an advanced hosted anti-spam service aimed at thwarting the underbelly of the web.

It is a very good spam prevention service which can be setup for your website. Setting up your Form with Akismet is very simple with LiveForm.

1. Register yourself on Akismet
2. Add a free or paid subscription from their home page.
3. Add your site to your Akismet account
4. Enter and save your Akismet key in LiveForm

Now, you are setup with Akismet. Every form submission from this form will be checked for spam using Akismet.