There seems to be a bug with the Meta Robots Advanced settings. It wont remove the settings when you select 'Remove', after pressing save they just reappear. If you select 'undefined' it doesn't save it, they just come back.
The only way to remove any is to select another tag, then remove the others ones, but then you are still left with the last tag you added. Its as if it wont accept saving if no tags are selected.
This is rather dangerous as you think you have removed a tag, but if you don't check after saving you wont know it still on.