OSpam-a-not protect your forms from spam.
To get started, follow these installation instruction for OSpam-a-not.
Next, go to Extensions > Plugin manager. Make sure the "System - OSpam-a-not" plugin is enabled.
Open up the settings for the plugin.
The key setting is "Minimum Form Time". If a form is submitted too quickly, the user may not be human. Set this to a minimum number of seconds that must elapse for a human submitter to fill out a form.
You can also choose to log blocked attempts through O-Spam-a-Not. Click here for logging instructions.