Hi Andre,
If you are using JBS 6.2.x you can use OSTwigits captcha plugin then enable it from the Template Display Settings - Study Details section.
Find the plugin here:
OSTWigits Captcha
If you are using JBS 7.x.x you will need to set up a public and private key on reCaptcha.
Find it here:
reCaptcha You enter the public and private key in the same area of the admin portion of JBS. You will need to create an account for reCaptcha.
Hope this helps.