Well, if anyone is interested, clearing "local data", as described in the link above, works - but only for a few moments, until the Vox's next automatic sync with the Kobo servers. Then all your existing RL awards are restored.
Creating another account also - as you might expect - clears RL data, but is pointless, because of course, you lose access to all the store-bought books you've downloaded on your earlier account, in spite of a vague suggestion to the contrary here:
(Notice this refers only to the deletion of side-loaded titles).
I did email customer services as an earlier poster suggested, but this has resulted only in a bizarre, robotic, we-can't-be-bothered-to-actually-read-your-message insistence that I restore my Vox to factory settings. Of course, this doesn't work, as all your awards are restored at the first sync with the Kobo servers. When I responded to their suggestion that I do a factory reset by pointing this out, they replied by...suggesting I do a factory reset. Arrgggh! If it's not possible to reset the RL awards, why not just say so?? I suspect I've gotten stuck in an automatic email loop or the responses come from some poorly trained tech support apparatchik in India who barely speaks English and doesn't understand my question.
Anywayz, I'll bounce it back to CS one more but I'm not optimistic. It does look as though this simply isn't possible without rebuying my library. I'm sure Kobo would love it if I did that but it's not going to happen...