We don't currently have a way of managing tags globally. If you add a document, blog post or thread and tag it with a tag that is incorrect, you can delete that tag by editing the thread / document / blog post. If that tag is only used in that one place and you delete it, it should be removed from the system.
I think I was able to verify this at the time, but I appear to have a problem on a 1.6 installation.
We have one incorrectly defined and set Tag. Deleting the tag, so that no tag instances appear in the Tag box. However, when saved the original tag is still present.
Interestingly, if you have more than one tag set or replace with another tag then the tag deletion works.
I suspect then a bug that won't allow deletion of a tag if it is the only one.