Silencing tracks

I have a recording which is in stereo that I want to clean up. The problem is, the left had track of my intro has noise on it and should be silent [ruining the intro of the track. I want to know is it possible to silence that left hand section on a stereo track. i know how to generate silence on a stereo track but not singularly. I have attached a screen shot showing the track and that the bottom section needs to be silenced. Please can anyone help.
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Probably the easiest way is to split the stereo track, then you can work on the two channels separately. See: http://manual.audacityteam.org/man/splitting_and_joining_stereo_tracks.html