Hi Philippe,
Thanks for contacting us and choosing our product!
About your problem with JUX Contact form, that's seem you are using our very old extensions.
You can you captcha in contact form by followings steps:
Step 1: Enable plugin CAPTCHA - reCAPTCHA in plug in manager
(You can get key from here:
www.google.com/recaptcha/intro/index.html
)
Step 2: Go to Global settings then choose Site Tab and choose Default captcha is CAPTCHA - reCAPTCHA
Hope this help!