Question: I'm using DSE Search 3.2.1. We have removed some unneeded indexes and fields and posted the `schema.xml` document to all of the nodes. Do we need to do anything else to have it discontinue indexing data? Do we need to run a reindex, or a full reindex?

Answer: 
I'm pretty sure from what I see in Solr you need to reindex after changing the fields in the documents in your solr schema.xml. After you post it, you'll need to reload the core. If querying still works after that you *might* be ok, but I would guess you're going to need to run a reindex to be safe.
If you don't reindex, the existing Solr index field values will remain, occupying space and responding to queries. And fresh inserts or updates will not have the deleted fields. As Ben said, that might be okay.

A Solr reindex will delete all of the old field values.