In Ext JS 4.x you can just clear the sorters on the grid before you filter.
grid.store.sorters.clear();
grid.store.filter("name", "Lisa");
Here is a working fiddle:
http://jsfiddle.net/Vandeplas/5aKdc/
UPDATE:
if you don't filter/make a change you can force the UI to update by using:
grid.view.refresh();