Although you see people saying "no comment" to the media on fictional tv shows and movies, in fact, no real public relations person worth their salt would ever say that.
Obviously, it doesn't mean "I will not utter a sound" but rather "I can't reply in any substantive way, so I will not answer your question."
In forums, it tends to be used sarcastically. If you say "no comment" your real message is "You're full of ... [bad word not typed]." It's actually full of meaning when used that way.