Unfortunately there is not a way to protect the USB stick beyond the bitlocker to go. Since the user has the password they can mount in their machine and have access to the files. Whenever someone has physical access there is always this type of issue.
You may want to look at a Remote Desktop solution instead if you need total control. Remote Desktop Services Overview https://technet.microsoft.com/en-us/library/hh831447.aspx?f=255&MSPPError=-2147217396