You can't, it's a user preference as of now.
Similar question is found here
From Hanselman
Unfortunately there is no checkbox or "just turn it off" way to disable the keyboard with a supported option. However, there is a way to effectively disable the keyboard by stopping the service that controls it.
Press the Windows key + W Type "services," and press Enter Scroll down to "Touch screen keyboard and handwriting panel" You can either right click and "Stop" or you can double-click and change it from "Automatic" startup to "Manual."