If students are not engaged, stop teaching. Wait for them to engage.
But if I'm always stopping and waiting, how will we get anything done?
Do you really think you are getting anything done by teaching while students are ignoring you? Actually you are accomplishing something, but it's not good: You are reinforcing the idea that students do not have to pay attention to you. When it's clearly optional to you, the decision to disengage becomes easy for them.
I don't want to be mean...I don't want students to dislike me or my class.
Look, my rehearsals are not silent. I am a fan of students asking clarifying questions of one another during rehearsal. I want students to enjoy making music, I don't want them fearing me, we even laugh once in a while ;-)
But when I need everyone to listen, I wait until everyone is listening. The more you do this, the less you'll need to wait, trust me. You don't need to be angry (and in fact you shouldn't be), you just need to be patient. The students need to know that you will not teach unless they engage. Having an expectation that students engage with you does not make you mean, it makes you more effective.
If you feel your teaching is worth it (and I hope you do), then don't try to teach a chatty ensemble. Wait for quiet, then teach your butt off.