Go to security settings and disable face unlock. If you want to be extra safe against Google, go to the advanced security settings, find the "trust agents", and disable the ability for Google Play Services to unlock your phone. That'll also kill any other unlock mechanisms you may have forgotten about tied to Google's services.

If unlock features remain after that, it's a manufacturer feature that's been set up. In that case you'll have to look for a guide for your specific brand and model.

Your phone can't turn this on by itself, if it's doing face recognition that means you set it up at some point.