Comment 18 for bug 366780

Quote from upstream report (https://bugzilla.gnome.org/show_bug.cgi?id=359556):

There won't ever be user-defined priorities in NM. NM connects to the last
network you've connected to that it can find. When you connect, NM timestamps
the network, and it will first connect to the most recent network you have
chosen. NM 0.7 also matches security settings, so if you have a WPA-enabled
'linksys' and an open 'linksys' saved it will pick the right one. 0.7 also has
a connection editor that will allow for easy removal of networks, but with
0.6.5 you can also use gconftool-2 --recursive-unset /path/to/your/network to
remove it.

If you dont' want NM connecting to a network, remove it from the network list
and dont' manually connect to it again.