While the Imam Delivers his Khutba
Narrated Abu Huraira: Allah’s Apostle (p.b.u.h) said, “When the Imam is delivering the Khutba, and you ask your companion to keep quiet and listen, then no doubt you have done an evil act.”
It is evil because you are adding to the disturbance the other person has already created. The idea seems to be that the person who is causing disturbance during khutba, should realize himself that he is doing something wrong by observing that every one else is absorbed in listening to it. However, if someone shouts to correct him, he will create further noise that would distract others and would in the process, also show that he himself was not attentive towards the khutba. However, signalling at the person creating noise to stop him, in my opinion, would not constitute evil.