Bot registrations even when Captcha enabled

I have bot registrations and i don't get how ... i have captcha enabled on the registration page but i can in logs the registration takes one second ... how do they (????) overcome it ? and what should i do to stop it ?

2013-05-24 20:08:48 10.159.54.236 GET / - 80 - 46.175.212.152 Opera/9.80+(Windows+NT+6.1;+WOW64;+U;+Edition+Next;+Edition+Yx;+ru)+Presto/2.11.310+Version/12.50 302 0 0 156
2013-05-24 20:08:50 10.159.54.236 GET / - 80 - 46.175.212.152 Opera/9.80+(Windows+NT+6.1;+WOW64;+U;+Edition+Next;+Edition+Yx;+ru)+Presto/2.11.310+Version/12.50 200 0 0 1076
2013-05-24 20:08:51 10.159.54.236 GET /register - 80 - 46.175.212.152 Opera/9.80+(Windows+NT+6.1;+WOW64;+U;+Edition+Next;+Edition+Yx;+ru)+Presto/2.11.310+Version/12.50 200 0 0 1029
2013-05-24 20:08:52 10.159.54.236 POST /register - 80 - 46.175.212.152 Opera/9.80+(Windows+NT+6.1;+WOW64;+U;+Edition+Next;+Edition+Yx;+ru)+Presto/2.11.310+Version/12.50 302 0 0 592
2013-05-24 20:08:52 10.159.54.236 GET /keepalive - 80 - 23.21.226.143 - 302 0 0 15
2013-05-24 20:08:52 10.159.54.236 GET /registerresult/2 - 80 - 46.175.212.152 Opera/9.80+(Windows+NT+6.1;+WOW64;+U;+Edition+Next;+Edition+Yx;+ru)+Presto/2.11.310+Version/12.50 200 0 0 624

a lot of people said the fix presented in this thread is pretty effective at stopping bots.

https://www.nopcommerce.com/boards/t/23292/robots-filling-out-captcha-field-is-that-possible.aspx

thx :)