Formal:
* Zwijg! (pronounced "zwayg") - This is the most direct and formal way to say "be quiet."
* Wees stil! (pronounced "veys stil") - This is another formal way, meaning "be still."
Informal:
* Hou je bek! (pronounced "how yeh bek") - This is very informal and somewhat rude, meaning "Shut your mouth!"
* Doe eens stil! (pronounced "doo ehnz stil") - This is also informal and means "Be quiet for a moment!"
* Zeg niets! (pronounced "zeg niets") - This is a little less rude than "Hou je bek" and means "Don't say anything!"
Other options:
* Maak geen lawaai! (pronounced "mak geen la-vaye") - This means "Don't make noise!"
* Spreek zachter! (pronounced "sprehk zakhter") - This means "Speak softer!"
The best option will depend on the situation and your relationship with the person you are speaking to.