> in the dataset, I can still see what random code was drawn for each participant.
You willl have to put() the NULL or FALSE value or the empty string.
put('D012', NULL, true);
or
put('D012', FALSE, true);
or
put('D012', '', true);
Please also note that reloading the page will probably display another code, because your put() will make the random generator to forget that it has already drawn one.