Simply deleting the file from DIR/mod/pending/ will do it. If you want to notify the sender, just send him/her an email. There is an easy way to get ezmlm-idx programs to do it for you: just wait and let ezmlm-clean(1) take care of it for you, once the message has timed out (number of hours settable within 24-240 in DIR/modtime; default 120).