I have a VirtualBox process hanging around which I tried to kill (-KILL/-ABORT) but without success. The parent pid is 1 (init).
top shows the process as D which is documented as "uninterruptible sleep".
strace shows up nothing.
How can I get rid of this? It prevents me from unloading the vbox kernel driver to load a newer one.
The D state basically means that the process is waiting for disk I/O, or other block I/O that can't be interrupted. Sometimes this means the kernel or device is feverishly trying to read a bad block (especially from an optical disk). Sometimes it means there's something else.
The process cannot be killed until it gets out of the D state. Find out what it is waiting for and fix that. The easy way is to reboot. Sometimes removing the disk in question helps, but that can be rather dangerous: unfixable catastrophic hardware failure if you don't know what you're doing (read: smoke coming out).
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.