You can either use .remove() on option elements:
.remove() : Remove the set of matched elements from the DOM.
$('#models option').remove(); or $('#models').remove('option');
or use .empty() on select:
.empty() : Remove all child nodes of the set of matched elements from the DOM.
$('#models').empty();
however to repopulate deleted options, you need to store the option while deleting.
You can also achieve the same using show/hide:
$("#models option").hide();
and later on to show them:
$("#models option").show();