Bouncing around the net I found a very elegant and neat solution, namely a honey-pot.
Its 5 years old, but the plugin works in the latest 3.1 release of moodle and many older versions too.
The readme on the site is slightly outdated. Here's what to do:
Howto : 1/ Create a account : http://www.projecthoneypot.org/ 2/ Get a quicklink for your site : http://www.projecthoneypot.org/manage_quicklink.php 3/ Get en apikey : http://www.projecthoneypot.org/httpbl_configure.php 4/
Install the plugin in the moodle/local directoryIn your moodle goto: Admin->Plugins->install plugins and upload+install the zip file from the Github site. 5/ Go to Admin -> Security -> Project Honey Pot, and write your informations on setting page 6/ Activity can be visible in report
Just set it up. Remember to delete any signs of recaptcha from your moodle. You can type: 'captcha' in the admin-search and delete all the api keys from recaptcha.