Have you downloaded the latest daylight savings time update? Make sure you update to 10.4.9 too.
If the airport is an actual apple airport station, go to applications> Utilities> Airport admin utility> and see if you can delete either more than one station or more than one location.
If that doesn't work, go to the same utilities folder, and go to the airport express setup utility, select your station, or network, and make changes to it, do not create a new one, there should be a choice between them. You should be able to delete them from there.
I am not on a mac now, so I can't tell remember what you see in system preferences, but try going into network or something like that.