remove default gateway

On linux
Show routing table

# route -n

Delete default gateway

# route del default gw 192.168.1.1 dev eth1

Delete a route

# route del -net 192.168.1.0/24 dev eth1

On Windows
Show routing table

> route print

Delete default gateway

> route delete 0.0.0.0 mask 0.0.0.0 192.168.1.1