I have a VirtualBox process hanging around which I tried to kill (-KILL/-ABORT) but without success. The parent pid is 1 (init).
Simple answer: you cannot.
Longer answer: the uninterruptable sleep means the process will not be woken up by signals. It can be only woken up by what it's waiting for. When I get such situations eg. with CD-ROM, I usually reset the computer by using suspend-to-disk and resuming.