How to remove the "This post is for paying subscribers only" box in Ghost post pages

After updating to Ghost v.3.39.x or later, you may have noticed a new element being automatically inserted into your post page template on paid posts.

It looks like this (the blue box):

This element is added by Ghost's new Portal feature. I think the aim behind this feature is to let any publisher using any theme to utilise the "paid-only" feature. Unfortunately on themes that already support this (like Substation in the image above) you get an annoying duplication of content.

Luckily, it's easy to hide this element from your posts.

Go to the Code injection page in your Ghost admin and paste the following into the Header section.

<style>
    .gh-post-upgrade-cta { display:none; }
</style>

This is simply CSS that just hides the element. Simple but it does the job.