Unused keywords from v8

Let me begin by saying that I have very few keywords, only half a dozen or so, but two of them are unused.
These two were there in v8 when I upgraded to v9 and remain unused. They appear in the keywords filter even when there are no images in the folder/collection that I am in.
I have tried assigning the same keywords with the new keyword tools and the filter promptly shows the number of images with the particular keyword. However when I switch to a different folder which does not have any images with that particular keyword assigned it still shows up in the keyword filter as zero.
Deleting the keyword assigned with the new keyword tool merely returns that keyword to zero and does not remove it from the keyword filter.
It would seem as if the original keywords are simply being duplicated by the new keyword tools and not being replaced by them.
How can I rid myself of these two unused keywords lingering from v8 ?
Why is there no option to purge unused keywords as there is in LR ?

Regards

IanW