Removing widget without using unregister_widget

Check the following answer, adapt the code and it will do what you want.
https://wordpress.stackexchange.com/a/3005/12615

Use Firebug for Firefox, or Chrome/Safari inspector to discover the widget ID.

Note the use of CSS3 attribute selector, that allows targeting an element using a partial ID or Class.
For example, this will hide the Calendar widget:
div.widget[id*=_calendar-] { display:none; }