Hello,
You have to note that when your product page is loaded, the dropdown option list is loaded at the same time, and so to have what you want, I see 2 solutions :
1° ) Instead of displaying dropdown, use Table or List in order to don't display unavailable configuration.
2° ) Contact one of
our Partner
to add a custom in order to get Ajax request that update dropdown value, but note that for lots of possible configuration this process can be a little heavy and so not very smooth...
Note that we have in our todo list this kind of feature.
Hope this will fit your needs.
Regards