Disable internet but keep LAN enabled Windows

1. Go into the control Panel, network and Sharing, Change adaptor Settings, right click Ethernet /Properties, IPV4
2. In the DNS settings, enter a DNS that doesn't exist 1.1.1.1 and 1.1.1.2 Save it, your LAN still works, Internet Wont.
3. In Control Panel - internet Options/ Connections Tab / LAN Settings - Put a tick In Use Proxy.
4. Enter something that doesn't work, 0.0.0.0  port 80 you'll block internet but retain LAN.

Reverse changes

1. Untick Use DNS & return to Detect Automatically
2. Untick use Proxy.