Convert Clip to Black and White

There are a number of ways to do this. As indicated by peterjohndean above, it can be done internally at the frame, clip, or whole movie level by simply desaturating the range you want affected. This approach has the advantage of not changing your output work flow. You could also use the "Export using QuickTime" option and apply the "Color Tint" filter using the "Black & White" pop-up setting. If you prefer, you could also incorporate a faded "Black & White" tint option using the "Film Noise" filter option along with the generation of "hairs", "scratches", and/or "dust" particles to give your masterpiece an "aged" look if desired.