$(function(){
$(':checkbox[name=flag]').each(function(){
$(this).click(function(){
if($(this).attr('checked')){
$(':checkbox[name=flag]').removeAttr('checked');
$(this).attr('checked','checked');
}
});
});

});

<input name="flag" id="agree" type="checkbox" checked="checked" value="0"/>  Agreed to
<input name="flag" id="reject" type="checkbox" value="1" />  Don't agree with