That's actually a planned feature for 6.1.0, lol. The new post "spoiler" feature is implemented in 6.0.0 but it's only designed for posts that were reported and marked controversial. In 6.1.0 I'm going to expand it so people can flag their posts as 18+ or Spoiler manually. What it does is suppress the display of a posts contents until a button is clicked. You can actually take a look at what it looks like in my below blog post.
www.joomlapolis.com/blog/kyle/19031-cb-activity-6-0-0-beta-3
For now though maybe you could just utilize some CSS to hide the media. That would be the minimal way to go about it. It should still allow the modal window popup to work as well. Below should work.
.galleryActivity {
display: none !important;
}
You could also blur the media. The below CSS should work.
.galleryActivity {
overflow: hidden;
}
.galleryActivity > * {
filter: blur(10px);
}
Adjust the blur filter size to increase the intensity of the blur. Blurring is probably the best approach here. It lets the visitor know there's something there, but it's obscured. Clicking it will show the modal window as usual. So it requires an explicit action to see the content.