If your emails are set to queue the only way I know of doing this is via the terminal.
You would need to ssh into your terminal (maybe you can do this from cpanel file manager as well - never tried) and navigate to the spool directory, usually sitting in /var/www/mautic/var/spool/
You will then need to remove the spool.
Funny thing with the spool is there are files that are created with a hyphen “-” at the beginning of the file name and the regular rm command does not like this, so what you will need to do is the following command
sudo rm -rf – -* [This will remove all the files that start with a -]
then you will need to remove the rest
sudo rm -rf *