Where in the Craft Control Panel can I globally delete a tag?
Not from a specific entry, but completely out of the system.
Currently it’s not possible to delete tags (or otherwise manage them).
I was searching for a way to do this as well, and as the answer indicates, it's not possible. However, by double-clicking on a tag you want to delete you can rename it to something else – so you can't delete it, but you can change it. You just have to remember when it comes time to create a new tag to rename that existing tag instead of creating a new one.
Delete the Tag Field in Settings > Fields.
More about Tags Fields: http://buildwithcraft.com/docs/tags-fields
EDIT
This answer is incorrect. The correct answer is:
Currently it’s not possible to delete tags (or otherwise manage them).