This way will clear the tags from a list (and books):
Pick a book with only tags you will KEEP.
Open the Metadata editor (blue i)
To the right of Tags, there is a button
Tag editor)
Select the ones you wish to 'bin'
use the recycle button on the left
Alternate: (and IMHO better because it will re-tag to something you wnt):
Use the Tag Browser:
Right click on an un-desired tag name:
'Rename' (
if to an already existing name: it will not double tag if you got the spelling and case correct)