It won't unlock if you don't look at the sensor- so you can either look away or close your eyes. This behavior can be enabled or disabled in Settings -> Face ID & Passcode -> "Require Attention for Face ID".
If you hold the sleep button and either volume button for a few seconds- until the power off/emergency call screen shows- it will disable Face ID and you'll need to enter your PIN.
If you hold the sleep button and either volume button for a few seconds- until the power off/emergency call screen shows- it will disable Face ID and you'll need to enter your PIN.