i solved it, actually the correct code is as follows:

#wpforo #wpforo-wrap .wpforo-profile-wrap .profile-stat-data-item:nth-child(3) {
   display: none;
}

but you put me on the right path 😀 

Use the CSS code below: 

#wpforo #wpforo-wrap .wpforo-profile-wrap .profile-stat-data-item:nth-child(2) {
  display: none;
}

The CSS code should be added in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Style Tab.  

Please don't forget to delete all caches and press Ctrl+F5 (twice) on the frontend before checking.

My checkboxes are already unchecked and, actually, secondary groups don't appear on posts.

But they are visible in the profile page (see image attached in the original post).

Can I make them hidden?

The option is located in the Dashboard > Forums > Settings > Members Tab. It's called "Display Secondary Usergroups under Post Author Avatar". You'll need to uncheck the checkboxes.