Setting the values of KBD_BEEPER_FREQ and CONSOLE_BEEPER_FREQ to 0 in /etc/default/kbd and a subsequent reboot did not solved the problem for me. But executing
xset b off
as normal user worked well. To deactivate the audible bell permanently, you have to
# pkg install SUNWgnome-config-editor
...and go to /apps/metacity/general/, where you have to uncheck the audible_bell property.