The best advice that I read on the Internet:
a) to think carefully about the limits you are setting. Do you really need the room to be dead quiet during Writing time? Maybe you do, and that’s okay, but make sure you know why you need it that way because one of the munchkins might want to know. In fact, if you tell them the reasons behind the rules, they’ll have an easier time respecting those rules, they’ll understand them better, and they’ll think about how their actions affect others. (For more on this see Alfie Kohn’s Beyond Discipline… ’cause I’m sure you have a lot of spare time for reading right now…)
And b) Stick to your guns. If you asked them to do it, you needed them to do it, and so there is a problem if they didn’t do it.