If you dislike the kibitzes, you can always 'toggle kibitz off', to completely shut them out. If you want the kibitzes to get more game-related, it often is no use at all to tell people off or to ask them to shut up, but asking them to switch to more private ways of communication like tell or channels often helps, and commenting on the game (kibitzing :-) or asking questions about it just may work.
Some special aspects to kibitzing exist, however. First, it is obvious that any player of a game has the possibility of mailing the game to him/herself afterwards, and reading the kibitzes thereof. Thus, keep in mind that the discussion that happens during the game is recorded and attached to the game, and please do take care not to make any comments that could be considered insulting ("not a real dan" is a typical example).
This especially applies to any pros and their games, please take care. Professionals come to IGS to give us, bloody amateurs, teaching on their own free time! If insulted, it's all too likely they'll go away for keeps. Just how would you feel if you were being slighted in something you're really good at by some rookie? To professionals, who have no need of an ego-trip in the field of go, kibitzes and especially shouts to the effect "not a real pro" or "fake 7p" are particularily insulting.