Ghost Blocks

This is most definitely an edge case, but here's the issue.

1. Install campaignmonitor.
2. Enter api and list keys.
3. Place list block (via blocks or context).
4. Change api and list keys.
5. Refresh list cache.
6. Observe that old block still appears with an error.
7. Via context menu, edit block and set it to appear in no regions.
8. No more errors.

We like to call it our ghost block :)

I'm assuming that either on api/list key update or list refresh a check for existing blocks should run and purge.