Deleting the system message on DirectAdmin panel

If you need to remove system message as an admin

cd /usr/local/directadmin/data/admin
echo -n "" > tickets.list

If you need to remove system message as a user

cd /usr/local/directadmin/data/users/username
echo -n "" > tickets.list

In case that you want to keep the 50 newest messages

cp tickets.list tickets.list.backup
tail -n 500 tickets.list.backup > tickets.list
rm tickets.list.backup

So, after above commands are run, All of the header messages will be deleted, however, the message data still exists.
In case that you want to remove it completely, you need to run following commands as well

cd /usr/local/directadmin/data/tickets
rm -rf 000000 000001 000002

For example command, the folder 000000 to 000002 will be deleted, to delete all of folders, you need to run

rm -rf *