When Microsoft has introduced Windows 8 they had replaced Start Menu with Start Screen which list all apps installed in your machine. When you install any new app from Windows Store then that app will itself to Start Screen just like any app pin itself to Start Menu in earlier Windows version.
If you want to keep your Start Screen minimal then here is a simple workaround which stop apps which have been downloaded from Windows Store to pin themselves to Start Screen.
Press Windows Key + R to bring your run command and type gpedit.msc and it will bring your Local Group Policy Editor.
Now navigate to Computer Configuration -> Administrative Templates -> Start Menu and Taskbar.
Now in right side pane you will find an option saying Pin apps to Start when installed double click it and enable it.
Now only those apps will be pinned to Start Screen when there AppId is included in the list, else they will not be pinned to your Start Screen.