(via www.brandonhutchinson.com)
Sendmail does not provide a command-line argument to remove messages from the mail queue. It may be necessary to manually remove messages from the mail queue rather than allowing Sendmail to attempt redelivery of messages for Timeout.queureturn days (5, by default).
The proper way to remove messages from the mail queue is to use the qtool.pl program included in the contrib subdirectory of the Sendmail source code distribution. qtool.pl uses the same file locking mechanism as Sendmail...