1st, the most important:
What captcha are you using on your site? Obviously if you use ReCAPTCHA, it’s vulnerably on 53% to 77% on bots, if is one comparative captcha or read color it can be more than 80% success using bots.
Try usin PlusCaptcha is an dinamic captcha sistem with blind radius and spot.
Here is the documentario (in Spanish, but its easy to install):
http://pluscaptcha.com/blog/instalar-pluscaptcha-en-wordpress/
Good Luck!
