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.
There is essentially no way to stop someone, somewhere from building a QC. Whether it is in the US, China, the EU, UK, Japan etc, someone, somewhere will do it. There is a ton of research on it going on and no way will it stop unless we move to some type of totalitarian system worldwide. Whether a QC will eventually be able to cause an issue for bitcoin is somewhat open to debate but the length of time to respond with upgrades etc is quite long which is why people are discussing it now.
Either with a QC or freezing coins the owner will no longer have access to them so it is just a question of whether they should be frozen or not. It seems to be a no-brainer that freezing is preferable to what would probably amount to a destruction of bitcoin if they are not. :-)