iMovie probably isn't the best choice for this. Because the movie file is already 'mixed down' it's hard to separate sounds, but it might be possible to minimise the unwanted sounds
If you can get the audio into an audio editing programme like Audigy or Logic I have the following thoughts:
1 - Stereo separation
Was the video recorded in stereo and where the people standing on opposite sides of the shot? If so you might find less of the unwanted voice in the left or right channel. You could therefore pan completely left at the point of the offending crossover, then bounce the audio file as a mono file.
2 - Tone / Frequency
Are the voices different in tone? By using a very narrow parametric eq you may be able to dial out the words by reducing the gain on the frequencies of the unwanted voice. In MS Logic the EQ facility has a function which allows you to see the frequencies of an audio file graphically.
3 - Relative volumes
Is the unwanted voice quieter? If so a noise reduction plugin might work to reduce the unwanted voice further.
4 - Repeated words?
Is the problem word said somewhere else by the speaker? If so you might be able to cut and paste the word from another part of his speech over the unwanted interruption
5 - Generally removing the other voice
Where they are not overlapping it will be easier to remove the other voice, just use the audio editor's tools to silence the audio file when the other person is talking
Once you've done all your audio editing you'll need to bounce and export the audio file then drop it back into iMovie, silencing the original sountrack.
Free tools you can use for all of this include Quicktime for separating the audio file, Audigy (for audio editing), or ideally Logic.
