JSON REST API is useful for developers, but many site owners find it unusable. The JSON REST API makes it easy to recover data using GET request which is useful for developers building apps using WordPress. But this could potentially expose your website to a new front of DDoS attacks and can be resource intensive slowing down the site.
How to easily Deactivate JSON REST API in WordPress
In your WP dashboard scroll over the Plugins widget.
In the plugin search box, type Disable JSON API.
Click on Install Now the Plugin Disable JSON API.
The Plugin can be activated by clicking Activate Plugin.
Once the plugin is activated it is all set for use as there are no settings to be configured. The plugin will deactivate the JSON API on your site.