Uninstall mariadb-server
To remove just mariadb-server package itself from Debian 9 (Stretch) execute on terminal:
sudo apt-get remove mariadb-server
Uninstall mariadb-server and it's dependent packages
To remove the mariadb-server package and any other dependant package which are no longer needed from Debian Stretch.
sudo apt-get remove --auto-remove mariadb-server
Purging mariadb-server
If you also want to delete configuration and/or data files of mariadb-server from Debian Stretch then this will work:
sudo apt-get purge mariadb-server
To delete configuration and/or data files of mariadb-server and it's dependencies from Debian Stretch then execute:
sudo apt-get purge --auto-remove mariadb-server
apt-get remove mariadb-client-10.1 mariadb-client-core-10.1 mariadb-common mariadb-server-10.1 mariadb-server-core-10.1