Philip John

Need to make sure that a site is private for all but the administrators? Just activate this plugin.
There are no settings, no configuration – it just forces anyone that isn’t an admin to go to the login page. They won’t see anything of the site at all. Great for hiding sites really quickly.

Simple Site Lockdown

Provides a really simple mechanism for locking down a site so that it's private to all but logged in admin users.

maxhaesslein

Block AI Crawlers directly via their IP addresses or user-agents. The blocking happens directly on the server, and not via robots.txt, so it should also block crawlers that do not respect the robots.txt.
This plugin may not work if you use a caching plugin that hooks before the ‘plugins_loaded’ hook. I may add compatibility in a future update.
After installation you need to activate blocking on the plugin settings page. The default settings should suffice, but you can add additional IP addresses, user agents or even JSON endpoints with lists of ip ranges. 
The visitor ip address server variable can also be customized, if your site is behind a reverse proxy and REMOTE_ADDR is not available.
If you want to delete all data the plugin saves in your database, activate the ‘delete all plugin data on uninstall’ option before uninstalling.

Simple AI Blocker

Block AI Crawlers directly via their IP addresses or user-agents.

Simple Site Lockdown vs Simple AI Blocker