You can edit the aspect ratio of any clip in its Media properties. Click the Gear icon next to the clip's name in the media panel to open its properties. Then un-tick the "From File" box next to the aspect listing, and use the menu to select a different aspect ratio, according to what you need.
Ah yes, thanks. I was confused by the numbers - wanting a "16:9" and not 1.46
It is possible that black pillars have been encoded on both sides of a squeezed anamorphic, and in that case you're hosed.
Nope, the setting Axel pointed out worked