Is there a way in Evernote to bulk delete unassigned tags?
It's not using the web interface, but Mac users can try this AppleScript which deletes unused Evernote tags!
On the Web interface, use CTRL+Click to select all the unassigned tags at once. Go to Edit and select Delete. You'll get a confirmation message asking if you really want to delete these tags.
If you use the Windows client, you can choose to hide all unassigned tags. They're not gone but at least you won't see them.