If your website is not ready for the new Gutenberg editor, install this plugin. It will automatically disable Gutenberg once it is released in WordPress 5.0.

The Classic Editor plugin is typically recommended for this, but it's not easy to set up. To get the same result as this plugin, you have to:

1. Activate the Classic Editor plugin
2. Activate the Gutenberg plugin (yes, you have to turn on Gutenberg to disable Gutenberg)
3. Go to Settings > Writing, and uncheck "Do not replace the editor", which is checked by default.

With this plugin, you simply activate it and Gutenberg goes away. When you're ready to use Gutenberg, deactivate this plugin.