This also works in the Mac version (as of April 2017). Click on tags, then click on Note Count to sort by count. Scroll all the way to the right to find your unused tags. Right-click/control-click to select Delete Tag. Beware that if the tag is from a Shared notebook it will give you an error message and warn that you can either remove the tag from notes or cancel. When I did this for a couple of tags, after the second tag, Evernote crashed. This was in Version 6.11 (454874 Direct).
I received a message from technical support regarding this problem. It said the development team is aware of and investigating the cause of this issue, they don't know when it will be resolved, and to keep watch for the latest news about it in this forum.