This plugin adds spam checks to catch contact form 7 spam. It does so in the background without annoying your legitimate users with captchas.
It adds a hidden field to the your contact form 7 form and then checks when a contact form 7 is submitted for that field. If its not present, or its filled in, then we can be sure that it was a bot of some sort.
It also adds the ability to check for links in the your-message field (assuming you used it) and if there are more than n links (where n is configurable in the admin area) then it also assumes its spam.
It also allows you to turn on / off email notices of blocked spam or legitimate message. This should be off under normal circumstances but its nice to test with initially to see that it works!