All apt repositories come from /etc/apt/sources.list and /etc/apt/sources.list.d/*. Assuming you added one via the MariaDB site, you can find that in /etc/apt/sources.list.d/ with the name MariaDB in it. Just delete that file and don't forget to remove the key from apt. You can do that by running apt-key list and then apt-key del <key id>. Then just refresh your packages and you should be all set!