Click on the "Tweak Settings" link under "Server Setup". Within the "Mail" section is a check box with the heading:
"Default catch-all/default address behavior for new accounts. fail will generally save the most CPU time."
You can chose from:
"localuser" (the owner of the domain)
"blackhole" (the email is silently discarded to the bit bucket aka /dev/null).
"fail" (the MTA that sent the email is sent a failure code for nonexistent users).
Tick the check box for the option you would like to use and apply the changes by clicking on the "Save" button at the bottom of the page.