How do I disable a network interface in Gutsy

[Rashkae]

In the network configuration tool, I can only uncheck "Enable Roaming" 
if I configure the interface to something manually.. How can I disable 
an interface so it doesn't connect to anything unless I explicitly 
enable it? )Without uninstalling Network Manager, if at all possible.  I 
have no problem modifying /etc/interfaces be hand if that is required.)