Is your goal isolation or just having the devices not access the internet?
If your goal is only to prevent them from accessing the internet, the simplest solution is to hardcode the IP addresses on your devices, but leave the gateway settings blank (or use a dummy address if needed)
If your goal is only to prevent them from accessing the internet, the simplest solution is to hardcode the IP addresses on your devices, but leave the gateway settings blank (or use a dummy address if needed)