Is the spam very specifically targeted at your site? If not, just implement your own very simple captcha system and see if they can handle that.

Spammers generally use captcha solving APIs which map to humans in low-wage countries. They pay ~$2/1000 solved captchas (a few years ago, not sure what it's like now.)

If you're not a specific target, changing your captcha might be enough to no longer easily be a victim of such a service without changes to the spammer software.