You could combine both of the above replies for a total bleep. First you can simply silence the offending word. Extract audio, show volume levels (in timeline view), find the starting frame, drag the clip volume down to zero, find the ending frame, drag the volume back to 100%. Lots of trial and error to find the best endpoints.
This volume adjustment now clearly shows you the location and duration for your "bleep." In iMovie, click Media, Audio, and there are hundreds of included sound effects. The iLife sound effects are categorized to help you find what you want. The Stingers category has lots of potential - I like the "cartoon boing". Or you could simply play a piano note in GarageBand and use that as your sound. Drag the sound you want into the second audio track, and there you go. Play around to find what fits, or shorten a long sound by dragging its volume down where you want it to end.
Endless possibilities. Have fun.