How to disable widgets PROBLEM

Firstly to mention that I am kind of new to wordpress, just installed it yesterday.

When I go to the widgets tab I can add(enable) them, but how do I disable them after that?

Here’s why I am asking:

When I finally found a theme I like i find out that it has a search bar which is great, but….The widget starts own search bar that doesn’t suit the theme at all.

I downloaded this plugin: http://wordpress.org/extend/plugins/disable-wordpress-widgets . It disables them and when I activated it, the search bar disappeared. But then when somebody tries to log in it returns “Fatal error: Call to undefined function register_sidebar() in /mounted-storage/home81b/sub003/sc44799-UKNH/www/wp-admin/includes/dashboard.php on line 12”.

Thanks.