I can’t find any good alternatives. I think anything done client side could be easily defeated.
This seems to be a known issue with capcha https://github.com/google/recaptcha/issues/87
It looks like the solution might be to use recapcha.net servers instead of google servers https://developers.google.com/recaptcha/docs/faq#can-i-use-recaptcha-globally
" Can I use reCAPTCHA globally?
Yes, please use “www.recaptcha.net” in your code in circumstances when “www.google.com” is not accessible.
- First, replace with
- After that, apply the same to everywhere else that uses “www.google.com/recaptcha/” on your site."