The message means that you was protected and avast! blocked the attack.
To turn off the display of these messages, go to the settings of the Network Shield provider and uncheck the option “Show warning messages”.
To turn off the IDS (not recommended), just terminate the provider and ask for ‘persist’ the changes after boot.
See a picture here: http://forum.avast.com/index.php?board=1;action=display;threadid=9078 or http://forum.avast.com/index.php?board=2;action=display;threadid=9367