You can delete a draft warning easily.
- Click on the polygon
- Choose to 'delete'
If you draft a warning and then don't require it please delete it when you are sure it isn't required to prevent the Public Information Dashboard from becoming 'cluttered'.
When you draft a warning it is 'locked' to you. This (helpfully) prevents other people editing your warnings as you work on it however it does mean it isn't possible for anyone else to delete draft warnings.