blocks appear in blockbuilder search results after underlying gist is deleted

I created and saved a block that I later wanted to scrap, but I couldn't find any way to get rid of it. Eventually I deleted the underlying Gist, but the block still shows up in [searches](http://blockbuilder.org/search#user=lwthatcher;d3version=v4), but trying to access it just results in a missing block error.

Even with existing blocks that I want to delete, there doesn't seem to be any option for me to do that.

Is there currently any way to get rid of unwanted blocks?

**Examples:**
- Deleted Gist: [Signal Plot IV (d3-tip)](http://blockbuilder.org/lwthatcher/3e61f97a6499ccfa77c85e8fe26bee19)
- Unwanted Block: [Delete Me!](http://blockbuilder.org/lwthatcher/d753f7ab84108d5fd9a7b6227d58a2cc)

Reproduce the search results using the deleted Gist and unwanted block links in the issue, then trace how search results are assembled and how block deletion is handled. Done means deleted or unwanted blocks no longer appear in searches and cannot produce missing-block results.

Written by the indexing model from the issue text.