You could take a look at wicd
as an alternate network management GUI..
Lenny install instructions are on the download page if you decide to try it out.
I prefer wicd to Network-manager after having tried both,
I do not know if it will resolve your issue or not..
Check out the features
and read some of the reviews..
Networks all have profiles, and theres a checkbox for "Automatically connect to this network" so I imagine if you create a profile for that device then Don't check the Automatically connect box it should ignore it..