Welcome to Android Central! Unless you fully powered off the Chromebook, it will always turn back on when you open the lid again. To fully power off, press and hold the power button until all windows close and the screen fades to black. Once powered off, it shouldn't power on again unless you press the power button--unless of course, Asus has some other function where opening the lid does in fact power it on.
If you logged off your account, or powered down completely, then when you start the Chromebook up again, you need to log into your Google account again. This keeps your data secure. If you only shut the lid (thus putting the device to sleep), then this shouldn't appear when you open the lid again.