What is recaptcha and how/will it help prevent spam ?
For answers to those questions you can look at google's page about recaptcha(https://google.com/recaptcha)
This installation guide requires joomla 3.4 (or higher).
- Navigate to the admin pannel of your joomla instalation
- Select plugins from the extentions pane in the top menu.
- Search for recaptcha in the search box
- Enable recaptcha by clicking on the status icon next to recaptcha
- One complete it is critical that you configure your recaptcha properly
- Go to recaptcah's configuration page by selecting recaptcha in the abovementioned pane
- make sure you select version 2 (default is version 1)
- Fill in the required keys you got on google's recaptcha administrator page
- Once done joomla wil begin to require recaptcha's