To delete one message from the exim queue
exim -Mrm message-idTo find the ids of all qued mails
mailq
If you wish to delete all the messages to a specific email recipient.
exiqgrep -i -r '<mail@example.com>' | xargs exim -Mrm
Of if you wish to delete all messages that were sent from a specific email address.
exiqgrep -i -f '<mail@example.com>' | xargs exim -Mrm