Using Id:
To Disable the Element:
jQuery('#id').attr('disabled','disabled'); //Here id is form element id
To Enable the Element:
jQuery('#id').removeAttr("disabled");
Using Name:
To Disable the Element:
jQuery('input[name="elementname"]').attr('disabled','disabled'); //Here element name is form element name
To Enable the Element:
jQuery('input[name="elementname"]').removeAttr('disabled');