I don't think you can delete a wireless network from the menu, but you can set up "locations" where you can set it up to connect to a specific network when you set your location. In the network portion of system preferences, you can edit your locations and which network it will automatically join.
But I think what you are wanting to do is change the "by default, join:" and if you click on the options next to it, you can change your preferred networks and the order you want them in.
EDIT: beat to it