Are tampons a good remedy for intractable diarrhea? If so, where do you put it?

Bad idea. Tampons are meant for uterine/vaginal bleeding. You insert it, it expands when wet, can't migrate farther up, and can be reasonably easily removed. Foreign bodies in the rectum can migrate up, beyond the reach for retrievel (unpleasant at best), and arguably could cause a bowel obstruction. See your doc, find out why you have diarrhea, and treat it the right way.