Sounds simple enough. Why can't I just say 'No comment' and leave it at that?
Here's why. Let me paint a scenario you don’t want to experience.
How 'no comment' can hurt you
Your phone rings. You pick it up.
“Mike Williams here from DXTV. We’ve just heard about your problem with the—”
“No comment.” Click.
That night, on the six o’clock news you see Mike Williams, standing in front of your office saying, “We put those allegations to the manager—that his building is a legitimate business by day and a mafia laundering operation by night. He refused to comment.” To the public audience, perceived unwillingness to talk = guilty as sin.
Using 'no comment' effectively
There’s nothing wrong with not commenting—often you must not comment, perhaps because it's still under investigation, or for legal reasons—what’s important is that you always say why you can't comment, and your tone and demeanour must indicate your normal willingness to be open when appropriate. It just happens that this time, it isn't.
A typical response might be, “I’m sorry, I can’t talk about that because there has been an arrest or it’s under investigation.” Or “No it wouldn’t be right to comment on that, it wouldn’t be fair to either side until the investigation is complete.”
One more point.
When a broadcasting reporter goes to an experienced interviewee and asks for comment the reply is very often like this. “No, I can’t comment on that because it’s still under investigation… do you want to record me saying so?” The recorder or camera is switched on, the question asked again, and the same answer given. “I can’t comment on that because...”