Any alternative to Googles reCAPTCHA?

Backstory we have a contact form, in a bootstrap modal, loading the form as an ajax request. The form has (as of today) a captcha, as we where getting a lot of spam.

Guess what it does on safari? Right... It renders outside the modal and since there's no need for scrolling, bootstraps modal adds an overflow hidden to the body. Results in non submitable forms on some resolutions.

Any idea on how to fix this, or other captcha systems we could use (it's a Symfony app).