Radek Mezulanik

You can lock whole website by setting it under maintenance.
Features:
<ul>
<li>
Select roles who applies maintenance mode
</li>
<li>
Set a redirect to login page
</li>
<li>
Edit a maintenance message in wysiwyg editor including media
</li>
</ul>
<h3>Sponsorship</h3>
<a href="https://www.productboard.com" rel="nofollow ugc">Productboard</a>

Zedna Maintenance Mode

Set you website under maintenance to lock content while development.

andyexeter

Post Lockdown protects your site-critical pages and posts by disabling all non-admin users’ ability to trash or delete them. 
It can also lock pages and posts, which will disable editing of the post as well as disabling trashing/deleting.
The plugin adds a new options page under the Settings menu in your WordPress admin panel which allows you to quickly search and 
select for all pages and posts of any post type. When you find the item you want to select, simply click it to move it to the right 
box and click Save Changes.
See the screenshots for an example of what an Editor would see when they view a list of posts with some locked and protected.

Post Lockdown

Works very well, my only criticism is that private posts/pages should be lockable, ie private posts should appear in the search form.

In order to lock them currently you need to publish them, then lock/portect, and then make it private again

Update!! the developer added the minor feuature request/bug. This plugin is now perfect!

Now perfect

I think the plugin is really useful, otherwise I wouldn't have made it ;)

Great Plugin

Allows admins to protect selected posts and pages so they cannot be trashed or deleted by non-admin users.

Zedna Maintenance Mode vs Post Lockdown