If you know site that's cheating you should post it in scam accusation.
What do you think this thread is? Did you just read the title and not bother looking at the post?
I don't know if 999dice is cheating this way, but I think even if they would you could avoid by setting client seed after viewing server seed's hash.
And for some time you don't need to click a button every time to see server seed hash, it's enough to open provably fair tab once.
You clearly did not read the thread. The server seed changes every time.