I don't think there's anything we can do aside from finding ways to make sure this doesn't happen. We can stop billionaire corporations from building quantum computers and the fact they assure us that they will do no sure thing isn't enough. We also can't rely on the government to regulate them.
I personally don't think its something to be scared about because for now it's beyond expensive to build a quantum computer and I doubt any company that manages to do it will involve themselves in such a risky act.
Building a quantum computer might seem a bit expensive. However, companies that aim to achieve quantum computers are supported by the government (or governments). If such a thing is possible, the government will definitely support it. Just like some hacking companies in certain countries whose goal is hacking wallets, exchanges, and other countries banks—all of them are backed by the government.