Since this is intended behaviour, there is no official way to do this? It ilustrates your users, that this is a drill down dimension and not a normal one- so they don't get confused if the dimension changes when they make a selection in the first dimension.
The only way to "hide" this is cheating around with CSS, i guess. But i haven't done this, so you need to find out by yourself.
If you found help, mark the correct answer and give some likes to ALL contributors, that tried to help.