This is disabling the internet for the lazy. The back of the computer is usually filthy and not very accessible, so here are three ways to accomplish this inside Windows.
1. Disable the Network Adapter.
Bottom right, Network and Sharing Center, Local Area Connection, and click disable:
Disabling the adapter is not always ideal in some cases; especially with VMs running.
2. Temporarily delete the default gateway.
Open the command prompt with administrator privileges.
Check the default route (0.0.0.0 mask 0.0.0.0). Remember the gateway.
Internet is inaccessible at this point. Add it back after.
route add 0.0.0.0 mask 0.0.0.0 192.168.0.254
3. ipconfig /release and afterward, ipconfig /renew