Remove all messages with a particulat string:

grep -lr 'a certain string' /var/spool/exim/input/ | \sed -e 's/^.*\/\([a-zA-Z0-9-]*\)-[DH]$/\1/g' | xargs exim -Mrm

Remove messages older than 5 days.

exiqgrep -o 432000 -i | xargs exim -Mrm