How to hide all categories except first in Block Extended Layout

This solution is tested on Block Extended layout.

Paste this code to Divi Theme Builder

.demo .post-categories a:not(:first-child){
display:none !important;
}

Then add this class name (demo) to Divi Blog Extras Module.

This will hide all the categories except first in the Block Extended layout of Divi Blog Extras plugin. You could try something similar for other layouts.