Pay attention to this guide because you could damage the machine
enable ssh access on qnap
Control panel -> Network & File Services -> Telnet /SSH
Enter in Qnap via SSH
Download Putty and login in qnap using name or ip address, username and password of administrator user.
Find your folder
All of you shares are under /share/<sharename>
cd /shares/myshare1/mydir pwd find . -type f -print -delete
The ‘pwd’ command will print the name of the directory you will be deleting the files from. This command will delete every file in the directory and print its name in your console (-print option).