[Question] Disable WP Instant-Publish - Allow Scheduled Posts Only

I have a friend with a WP blog that prepares his posts ahead of time. It's set up to post to FB/Twitter/etc. when there's a new post.

He's accidentally clicked "Publish" a couple of times. This is a pain in the ass, because then he has to go to each of his social media accounts and erase the incorrect post.

Does anyone know of a plugin or a setting that can disable (or warn the user when) clicking "Publish"?

I'm not very good with WP; obvious advice is welcome.

Thanks,
-bigot