I pulled this from the sencha API docs for Ext 3.4. This is the method that fires when you click a column header:
Code:
// private onHdMouseDown :function(e, t){if(t.className =='x-grid3-hd-checker'){ e.stopEvent();var hd =Ext.fly(t.parentNode);var isChecked = hd.hasClass('x-grid3-hd-checker-on');if(isChecked){ hd.removeClass('x-grid3-hd-checker-on');this.clearSelections();}else{ hd.addClass('x-grid3-hd-checker-on');this.selectAll();}}}
So, call that function on your header with a dummy Ext.Event object or get the header reference and just .removeClass('x-grid3-hd-checker-on') on it...
So probably just a Ext.fly to your header cell, then .removeClass on the aformentioned css class should do the trick. Happy hunting!