To lock the remote screen execute these commands on the remote windows machine:

reg add HKLM\Software\Policies\Google\Chrome /v RemoteAccessHostRequireCurtain /d 1 /t REG_DWORD /f && reg add "HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server" /v fDenyTSConnections /d 0 /t REG_DWORD /f && reg add "HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp" /v SecurityLayer /d 1 /t REG_DWORD /f && reg add "HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp" /v UserAuthentication /d 0 /t REG_DWORD /f && net stop chromoting && net start chromoting