How do I prevent source engine based games from adding the English US keyboard layout?

ATTENTION!!! The solution renames the registry keys for english USA layout!!!

Use only if you know what you are doing!

First backup your registry!

Execute in PowerShell:

Rename-Item -Path "Registry::HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Keyboard Layouts\00000409" -NewName "00000409.bak" -ErrorAction Ignore;
Rename-ItemProperty -Path "Registry::HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Keyboard Layout\DosKeybCodes" -Name "00000409" -NewName "00000409.bak" -ErrorAction Ignore;
Rename-ItemProperty -Path "Registry::HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Keyboard Layout\DosKeybCodes" -Name "00000409" -NewName "00000409.bak" -ErrorAction Ignore;

You can switch layout using win+space. alt+shift will still use ENG USA when switching although it will not be displayed on the taskbar.

After some Windows updates may need to re-execute commands.

It can be considered only a temporary solution until you find a more correct one.