Captcha was recently cracked and too many automated scripts,bots managed to get through it so i won't really depend on captcha until Newer version Shows up also do ask your hosting to updated PHP to latest available cause there was some PhP bug found last month which enables servers exploitations and enables redirections issues .

Best is find a plugin which gives you easy way to enable human question answer verification system where you can manually set questions and answers so it will be hard time for Automated scripts and bots to bypass through .