If you enable a setting on a index you cannot remove it.

For example if you want to disable shard relocation in cluster and enable it for one index you use:

curl -XPUT http://localhost:9200/articles/_settings -d '{"index.routing.allocation.disable_allocation": false}'
curl -XPUT http://localhost:9200/_cluster/settings -d '{"transient":{"cluster.routing.allocation.disable_allocation": true}}'

Then if you want to use the cluster setting on the index there is no way to delete the defined value and you need to manually maintain it.

A good enhancement will be to have the ability to use:

curl -XDELETE http://localhost:9200/articles/_settings -d '{"index.routing.allocation.disable_allocation"}'

or

 
curl -XPUT http://localhost:9200/articles/_settings -d '{"index.routing.allocation.disable_allocation":default}'