There's already some good answers here, just wanted to share my "solution"
I had a similar problem where I wanted some specific device to not have internet connection but still be on LAN. My lazy ass solution was to manually set the network settings with default gateway set to 0.0.0.0. With IPv6 it would be [::].
But this is not meant to be an advice. You may not even be able to manually set network settings on some of those IoT devices, and removing default gateway from DHCP server is not exactly an elegant solution. Perhaps you could set it to serve different settings based on MAC, but then the other solutions are perhaps simpler and better in some other way.
As I said, this is not an advice.