If receiving emails to your mailbox that are sent to non-existent mailboxes on your domain, you can control the activity with the following :

1) Login to Cpanel and go to Default Address.
2) Click on Set Default Address link.
3) Type :fail: in the textbox so that the emails NOT addressed to existing mailboxes will get bounced. Else you can set it to :blackhole: to permanently delete the messages.