Removing sidebar widgets

I want to unregister some of the default sidebar widgets that conflict with my design so they won’t appear in the admin panel and can’t be used. I make it for someone else, so just leaving them there in the admin panel and tell him not to use them doesn’t resolve it.

Based on that I tried writing in the functions.php in my theme:

unregister_sidebar_widget('Search');

And:

unregister_sidebar_widget('search');

Tried:

unregister_widget_control('search');

Tried that as well. Nothing works. The widgets still appear both in the admin and the website.

Is there a way to get rid of them? Should I edit the core files? And if so which one do I have to edit to make them go away from the admin panel?