Use this command to parse through the sendmail mailqueue looking for mail that matches a pattern. Generate a text file full of 'rm' commands to delete them.
find /full/path/to/dir -name 'qf*' -exec echo grep -i 'junk@mydomain.com' {} \> /dev/null \&\& echo {} \; | sh | awk '{s=$0;sub("qf", "df", s); print "rm " $0 " " s;}' >> deleteList.sh

chmod u+x deleteList.sh

./deleteList.sh


The output will be something like this:
rm /usr/var/spool/mqueue/qflB9NG2L9092080 /usr/var/spool/mqueue/dflB9NG2L9092080
rm /usr/var/spool/mqueue/qflB9NHrfn092354 /usr/var/spool/mqueue/dflB9NHrfn092354
rm /usr/var/spool/mqueue/qflBA1o0cd022723 /usr/var/spool/mqueue/dflBA1o0cd022723
rm /usr/var/spool/mqueue/qflB9NGRC5092128 /usr/var/spool/mqueue/dflB9NGRC5092128
rm /usr/var/spool/mqueue/qflBAJeBB7081428 /usr/var/spool/mqueue/dflBAJeBB7081428
rm /usr/var/spool/mqueue/qflBBMvLjB080827 /usr/var/spool/mqueue/dflBBMvLjB080827
rm /usr/var/spool/mqueue/qflB9NGtwk092210 /usr/var/spool/mqueue/dflB9NGtwk092210
rm /usr/var/spool/mqueue/qflB9NGmxf092189 /usr/var/spool/mqueue/dflB9NGmxf092189
rm /usr/var/spool/mqueue/qflB9NGtIn092211 /usr/var/spool/mqueue/dflB9NGtIn092211
rm /usr/var/spool/mqueue/qflB9NGS2d092133 /usr/var/spool/mqueue/dflB9NGS2d092133


I found this in the comments on a blog a little while ago.

Cheers,
Dan