To delete one message from the exim queue

exim -Mrm message-id

To 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