You can unlink the audio and video in the Timeline by selecting (highlighting) the clip and hit Command l (lowercase L). If you don't want the audio there in the first place, don't edit it into the Timeline to begin with.
If your video does not fill the Canvas, it could be due to a few different things:
A. You're using video with smaller pixel dimensions that your Sequence settings
B. You've sized the Canvas window smaller than 'Fit to Window'
C. You've scaled the clip lower than 100%
-DH