How can I remove audio for a section of a video/movie I imported from my camera?

There are a couple of ways to do this. One is with keyframes and the other by splitting the clip. To use keyframes add 2 keyframes (option click) where you want the audio to stop and 2 key frames where you want it to return. Then drag the audio bar down to the bottom of the clip. T place the keyframes put the playhead over where you want the keyframe and then option click. You can adjust the position by clicking on the key frame to select it and dragging it. Perhaps easier is to put the playhead over where you want the audio to stop and split the clip. Move the playhead to where you want the audio to return and split the clip again. This will give you 3 clips and then drag the audio down on the middle clip or mute it from the tool bar.

What version of iMovie are you using?