To whipe the Exim message queue use the following command:

exim -bp | exiqgrep -i | xargs exim -Mrm

Adding the following line to the Exim4 configuration prevents that all the disk space is used up by the messages in the spool folder:

check_spool_space=100M

This refuses incoming messages with a "452 Space shortage, please try later" message when less than 100 megabytes of disk space are available on the partition where the spool folder resides.