Our Gemara on Amud Aleph teaches us that the rabbis decreed that “One does not accept converts from the Tarmodim.”
Likutei Moharan (59) tells us that there are some people who are so evil that it is forbidden to draw them close to Judaism and bring them into service of God. He cites the mixed multitude that Moshe accepted from the Egyptians, who only caused trouble. God blamed Moshe for naively accepting their bid to join the Exodus. This kind of idea, that there is irredeemable evil, lands hard on our modern ears. We like to believe in equal opportunity for everyone and no one is completely a victim of their upbringing or current status.
There is a fascinating dispute in regard to the Amalekites, who from a Biblical and rabbinic perspective represent pure irredeemable evil. The simple understanding of the verses in the Torah (Devarim 20:16, 25:19 and and Shmuel I:15:3) is that we should show no mercy to Amalek and destroy them to the last person. Yet Rambam in laws of Kings 6:4 indicates that if the Amalekites or the Seven Nations respond with a bid for peace we do not attack them, rather they become serfs under a Jewish protectorate. Ra’avad (ibid) cannot abide by this and suggests, albeit with some implied reluctance, perhaps the Rambam meant if the Amalekites did a full conversion to Judaism. Regardless, this is not the simple reading of the Rambam.
We see from this dispute that it is a debatable point if any group or culture can descend to such a level of evil that we cannot accept good faith expressions of peaceful and cooperative intentions.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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