I don't have a 740, and from discussions about this subject in other threads it's clear that 740s are different in this regard compared to my 720. I have never actually seen a good explanation for how to permanently remove previously-stored network credentials on the 740.
On the 720, you would identify the "known" networks by seeing them listed with "(Pre)" in the name. This part I think is the same between the 720 and 740.
On the 720, if you're out of range of the "(Pre)" network, it may only appear for a few seconds after startup and going to the Network settings page, until the device completes its scan for nearby networks. You need to select the "(Pre)" network you want to delete, then click "Delete". Apparently, on the 740, there is no "Delete" option, but there is a "Disconnect" option. If I understand correctly, the "Disconnect" option does not permanently remove the stored network credentials, but at least during that powered-on session, the 740 will no longer try to connect to the "Disconnected" network. (So I think you may have to go through something like this process every time you start up the device? (shrug))
Maybe Brolin or someone with a 740 who has managed to overcome similar issues can provide better details.