Keeping them from "Playing" on the Computers During Class

Since so much of our course is computer-based, I realize that taking them away from the computer can be counter-productive, but I know that they are tempted to "play" while I am talking.  During my first class meeting yesterday, I had them just turn off the screens while I was talking and allowed them to turn them back on when we were walking through something together.  I walked up and down the sides of the room (because it is too difficult to get up and down the aisles) and all screens were off so it seemed to work!  It was nice not hearing the keys clicking while I was talking :)