Disable account creation (not just via access question)

I'd like to disable account creation via the web interface. I know about the access question, and I currently use it, but I'd prefer to fully disable account creation, rather than just making it probable that a random user can't create an account.

As far as I know, it's not possible to disable account creation (according to mailing list discussions from ~2010). If this is still the case, I'd like to request that this feature be added.

Many thanks for a great tool!