How to clean completely select2 control?

Why all this trouble???

use:

 $('#sel').select2('data', null);

Whenever you alter or change a value in select2 try to use the trigger ("change"):

$('#sel').empty().trigger("change");

Try this from official select2 documentations.

version 4

$("#sel").val(null).trigger("change");

For version 3.5.3

$("#sel").select2("val", "");