In Catholicism, how can Saints be de-canonized?

I was just reading about Clement of Alexandria on Wikipedia , which says that in 1586 he was removed from Roman Martyrology by Pope Sixtus V. I have never heard of another instance of a figure being removed from veneration. Is this an occurrence that has happened more times? If so, what is the process by which the martyrology can be amended by removing veneration from a figure?