Grey out other checkboxes
This question is not exactly related to drupal.

Still you can use jQuery to make it work.

Just get the wrapper class of parent element

jQuery('#edit-field-hobby-und .form-checkbox').click(function(){
  jQuery("#edit-field-hobby-und .form-checkbox").each(function(){
  jQuery(this).attr('disabled',true);

  });
  jQuery(this).removeAttr('disabled','disabled')
})


Here form-checkbox is the class of the checkboxes and edit-field-hobby-und is the id of its wrapper.

Hope it will help!