ahmedimraan

This plugin adds Custom Fields to WP REST API JSON output. Works with Advanced Custom Fields. 
Its as simple as Installing and Activating, once activated you can see an object named metadata.

WP REST API POST/ PAGE CUSTOM FIELDS

Adds Custom Field output to the WP REST API for posts, pages, users, and taxonomies.

arminmeth

While WordPress is slowly but surely becoming a complete application framework and JSON REST API is huge step in this direction, there is a large number of WordPress users that do not need advanced features like those offered by WordPress JSON REST API. This plugin helps them to turn those advanced features off and not worry about it.
While other similar plugins disable JSON REST API functionality only for anonymous users, Kill JSON REST API completely disables the framework for all users and removes links and tags from head element.

Kill JSON REST API

Completely disables JSON REST API for both registered and anonymous users in WordPress 4.7.* and removes API links and tags.

WP REST API POST/ PAGE CUSTOM FIELDS vs Kill JSON REST API