The original idea is from a hackerspace member.
A fake Wi-Fi access point, with a number as the SSID. Is a device connects to it, it increases the number and drops the device off the nerwork.
This script can run on any device that runs OpenWrt. Tested on an Onion Omega2+.
Upload the freewifi.sh to the device that runs OpenWrt and run it.
chmod +x freewifi.sh
./freewifi.sh
If you want this to run at boot, just run the next lines.
Make sure /mnt/freewifi is a writeable folder, or change the file variable in the setup.
cp freewifi.sh /etc/init.d/
chmod +x /etc/init.d/freewifi.sh
echo -e "sh /etc/init.d/freewifi.sh \nexit 0" > /etc/rc.local