Regressive was a semi-derogatory term the Enarans applied to a minority of their population who rejected all but the most primitive technology. They eschewed such things as communication interlinks, microfusion generators, or radioseptics. The Regressives claimed to be passive and not harming anyone, but according to the Enarans they just pretended, and spread lies, lacking a conscience. They were also believed to cause dissent and doubt, and have a destructive influence. Furthermore, they were said to be impossible to understand.
Mainstream Enarans believed they had been tolerant of them for a very long time, although they were segregated in their own villages. They wore identification devices around their necks which other Enarans didn't have.
Ultimately even this arrangement was considered insufficient, and they were subject to a "voluntary resettlement" in a new colony. Fear of their refusal of modern hygiene causing disease was a factor, as was the perception that they were undermining and holding the Enarans back in any way they could. Regressive "criminals" were described as a growing threat to society.
It was a gradual effort, spread over at least two months. Lists were produced, with people then rounded up and soldiers. People who got resettled were not heard of again. They shared stories that the transports didn't go anywhere and that the passengers just got vaporized in some kind of thermal sweep. Thousands dissipated this way, and the Regressives believed they were being slaughtered.
After all Regressives were removed, Enaran history recorded that they were the ones who wanted to leave, and that they had died due to violence and disease at their new home.
Korenna Mirell was witness to the fact that this injustice was being committed, but like most Enarans she denied the truth. Many years later, a dying Mirell transferred her memories to B'Elanna Torres, in the hope that one day the Enarans would have to own up to their actions. (VOY: "Remember")