What kind if switch/router are you using? You should be able to set up a firewall rule to block a device from accessing the internet. Of course, the steps depend entirely on your hardware and router OS, but essentially:
- give printer a static ip on you main network
- tell firewall that any traffic originating from printer static IP on the internal network destined to the ISP is not allowed, and vice versa
- assuming printer mac address doesnt change, you can also apply the same rule to the mac address.