If you want to reset the e-mail acount of a DirectAdmin account it’s pretty simple, you can just hit the reset button. But there are no such buttons for the e-mail accounts itself. So you can use this little line in SSH to remove the e-mail sent acount of a particular e-mail account:
rm -f /etc/virtual/domain.com/usage/info
Notice this line concerns firstname.lastname@example.org. So you have to change the domain.com part and the info part.