how can i disable blocking after failing login?

Hi,
i use the sidebar login for my site.
-when i try to login, i see the error that tell me:"the security code you entered is not validat .....". what is this? which security code? how can i solve it?
-you know if the user types username or password incorrectly for several times user’s account will block.
how can i disable blocking after failing login?

Regards
AHHP