Preferences > Customization > Editing > Polyphonic Velocity Audition. Uncheck it. You should only hear the one note you click. Incidentally, it would be logical to expect that unchecking the Velocity Audition checkbox above it would completely silence editing, but it only silences velocity adjustments. You still hear the one note you click when changing pitch or time. If you don't want to hear anything at all, the easiest way is to set a Hotkey to toggle the audio engine, and toggle it off while editing. It'll come back on immediately when you start playback to hear your changes.
There may be differences between the way all this works with soft synths vs. hardware outputs, and there may have been issues with these setting in the past, but I believe they work as advertised in X3 and Platinum. I haven't run into the problem with polyphonic cacophony for a long time with either software or hardware synths.
EDIT: Actually, I just checked this functionality in different versions, and it appears the Polyphonic Audition option hasn't worked since X1, so I guess OP is on X1...? Explains why I haven't had to deal with it for a long time... good riddance!
