Even if it's just a tiny computer - as soon as he hangs on the Internet, it is at risk and may be subject to attacks.
To avoid that you would have look through every log file, out from which IP the attack ran out and then lock this IP by hand using iptables as beschriben in this post Reduce Spam in Wordpress by blocking IP-Addresses with iptables
This time it is completely simple: All you need to do is nothing!
The package OpenSSH, which is installed by default in almost all Linux distributions and that is responsible for the fact, that you can log in to your banana via SSH, contains already a SFTP server. SFTP = Secure File Transfer Protocol, that means an encrypted FTP protocol.
That means, users who are set up on the Banana Pi can immediately connect the banana with a FTP client such as FileZilla. You just have to take care, that you specify "SFTP" as protokoll.
This requires a LEMP installation, as described here: Nginx, MySQL and PHP
This manual is written for an banana Pi, but it should also work for a Raspberry Pi or Beaglebone Black.
Step 1: Set up Nginx with SSL
These create the directory /etc/ssl/nginx, change to the directory and create a self-signed certificate.