Just like the groom who slandered his bride could never divorce her (in Mitzvah #554), the rapist who was obligated to marry his victim was likewise not allowed to divorce her. The reason for this is because of the way he abused her. He’s done enough to her and is not permit to degrade her any further.
If a Levite or an Israelite violated this mitzvah and divorced a woman he had raped, he was obligated to re-marry her. Since a kohein was not permitted to marry a divorcee, if he violated this mitzvah he was unable to re-marry her. In such a case, he would be flogged.
This prohibition applies in all times and places. It is discussed in the Talmud in tractate Kesubos (39a-40a) and codified in the Shulchan Aruch in Ever Ha’Ezer 177. This mitzvah is #358 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos, though it is not listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.