How do you stop people from asking and answering their own questions en masse?
By design, it's always been OK to ask a question and answer it yourself. One of the first SO questions I ever asked I did exactly that.
That said, over time, the social mores have changed a bit, and it's at least considered better form if you mark your own answer as community wiki such that you don't get rep from it.
In my opinion, this isn't a big issue. Other answers can come up and be just as good, and voted just as highly. It's not like the OP posting his own answer prevents others from posting theirs. And if theirs are better, the community will reward them.