Unknown User Registered into my system

Question

I'm currently doing recruitment system by using drupal 7.. In my system, the user (anonymous) need to register before they can log in into the system. For log in and registration, i use LoginToboggan module. i also use the captcha module to avoid from spam. However, when i look at the people..there is a lot of unknown user.. and it seems like spam. what should i do?