Go to cPanel, then "Mail", then "Default Address", then "Set default Address".
Make sure the in the domain after "Send all unrouted mail for" is correct, then fill in the next box (after "to:") with either ":blackhole: or ":fail:" or the email address to which you want unrouted mail sent. Then press the"Change" button.
(I find that a Gmail address works well, for filtering, since their SPAM filters are pretty good, but :blackhole: is best if you are sure you never want to see anything that arrives there).
I'm not sure what you are doing with
but, these are AFAIK not what we refer to as default addresses--unless your domain is something like default.ca or anything.com and therefore your username is "default" or "anything".
The default account is of the form:
your_username@your_domain.com
allen