ućutati

1. to fall silent; to stop talking
   Kad je ušao direktor, svi su ućutali. — When the director walked in, everyone fell silent.
   Ućuti više, ne mogu da te slušam! — Shut up already, I can't listen to you!

The imperative "Ućuti!" sounds rude, like "Shut up!". For 'to keep quiet about, not react' use ćutati or prećutati: Nisam mogla da ćutim na takvu nepravdu; prećutala je uvredu.