After playing with ReFS a bit, time came for Windows update restart. After initial second or so of booting screen blanked and stayed that way. I waited and waited but there was neither screen nor disk activity. And same thing happened every time I rebooted.
And it got frozen at the worst spot. I have a dual boot and freeze happen to be just before OS selection screen. With one error both installations were dead. And since I used BitLocker on both I wasn’t really in the mood for recovery.
Fortunately, after turning off system few times during boot, I got into Automatic Repair prompt. I entered BitLocker key for my main OS and proceeded with Advanced Options. There I just selected Continue (Exit and continue to Windows 8.1). While this should have technically been exactly the same boot, it was different enough that it actually worked. And then, as soon as system booted up, I removed ReFS capability (this registry file) and rebooted again. All came back successfully.
PS: Even when ReFS functionality is “turned off” you can still use existing ReFS disks. You just cannot format new ones.