I have no experience with sendmail as I only use qmail on my servers. The following site might be useful, although I cannot say one way or the other whether or not it is a good or accurate solution. The site was last updated over 2 years ago:
http://www.brandonhutchinson.com/del...ail_queue.html
On that page scroll down to the section titled "Removing mail by email address" - that script is for removing mail based on the address receiving it. Maybe you could modify the script to instead remove mail based on the sender.
I highly suggest testing this out on a development/testing server before running it on your production box.
