Widgets won’t go away even though plugins are removed.

Certain widgets belong to plugins are still around even though I remove the plugins completely from server. How? When you go into dashboard and under Appearance, go to widget and you see the widgets are there. The widgets are functioning correctly too when adding them to sidebars of blogs.

How to remove those widgets so it won’t be a bother for next update of wpmu — in case these widgets may present future problem for new wpmu’s versions?