You can use captcha security with DJ-Catalog "Ask about this product" form.
This tutorial is about core Joomla ReCaptcha functionality, other plugins may work different and may need different setup
Here's the short tutorial how to do it.
- enable the core Joomla captcha feature in Global Configuration
- open the captcha plugin
- generate the re-captcha keys at http://www.google.com/recaptcha/whyrecaptcha
- enter the keys in the plugin settings