A noise gate can easily solve this problem by only allowing sound through when the input volume reaches a certain level – i.e. when someone is talking and only then. My favorite audio swiss army knife for the Mac, Audio Hijack Pro, allows you to implement a noise gate very easily. Follow the simple steps in this video and the next time you're on an interview podcast (or recording one), you'll sound fantastic!
Podcasting How-To Video: Creating a Noise Gate on Your Mac
