How to persistently remove default gateway

[Jaime]

Hi.

Can anyone point out the recommended way to remove the default gateway
from my routing table? I'd also like this removal to persist across
reboots. I've tried using Gnome's "Network settings" application
("network-admin") but I can't work out how to use this app to remove the
default gateway (nor can I work out how to get *any* changes I make
using network-admin to persist after a reboot).

Many thanks,

Jaime