Hi tomvanarken – what theme are you referring to?
If it’s Boardwalk, this custom CSS will hide the featured image on single posts:
.single .entry-thumbnail {
display: none;
}
You’ll probably need to make other CSS tweaks as well to account for the fact that you’ve hidden the entire featured image area, which also contains your post title and date.
Don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.
If you need further help, could you please confirm which theme you’re talking about and provide a link to your site if possible.