If you're using a cable/sat box, often times these settings can be programmed in the box's output menu.
If you're using a straight co-ax, unfortunately it may require a toggle if the broadcaster is sending the signal as a 4:3 or 16x9, and they vary from channel to channel.
There is no option for the individual channel, at least as far as setting an aspect ratio, in the television menu.
I wonder if there is a way to set 40B750 to remember to display certain TV channels as 4:3, and others as 16:9 ? So that I don't have to switch