You could just disable the tag on each picture the tag is available.

The tag will then magically disappear!

The idea behind this is that if a tag isn't used, it will automatically be deleted. That means if you want to use that tag again, you just have to enter it again.

Oh wait, I was a bit too fast on that one. I just clicked around and I got my tag once to disappear, but that was probably just because I didn't press done. It appears that you really can't remove a tag very easily. I'll further look into this.