A MEDITATION ON THE LEGEND OF AHER (Elisha ben Abuyah) is cited by Rabbi Yair Hoffman in the Five Towns Jewish Times in an essay on responses to morally fallen Torah scholars in Jewish tradition:

In conclusion, there is a fascinating Kochvei Ohr which discusses the sin of Acher – Elisha Ben Abuyah - the teacher of Rabbi Meir and the sage who fell. While talking to his teacher who went off the path of Torah, Acher the teacher told Rabbi Meir that he had reached the Shabbos boundary. Rabbi Meir asked him how he knew. The teacher said that he was counting the footsteps of his horse. Rabbi Meir asked Acher to come back and return to Judaism. Acher responded that he could not because he was privy to a Bas Kol that said, “Return you wayward sons all except Acher – who knew my Glory and rebelled against Me..”
Rabbi Meir tried to convince him to come back anyway. Rav Yitzchok Blaser – the author of the Kochvei Ohr and student of Rabbi Yisroel Salanter asks two questions: The first is that how could it be that Hashem did not want the penance of Acher? The second question he asked was how it could be that Rabbi Meir is going against a Bas Kol – a direct request from Hashem Himself?
The Kochvei Ohr answers that generally speaking Teshuva is always somewhat tainted. We know that Hashem will accept our Teshuva ultimately if we offer it and He will forgive us. Even though it is tainted in this manner, nonetheless, Hashem does not care and accepts it anyway.
This is true for most people. However, Acher was so depraved and had fallen so far that he needed a completely perfect Teshuvah. He required a Teshuvah untainted by the knowledge that it would be accepted.
Hashem wanted Acher to perform this type of Teshuva and that is why Acher heard that Bas Kol. Rabbi Meir was aware of this, while his own teacher was not. He tried convincing him to do real Teshuvah anyway – even though it would not have been accepted.
We see from the words of Rav Blaser how very much Hashem wants everyone to do Teshuvah. May all of us be zocheh to a complete Teshuva in all that he do wrong. Amain.

I had never heard this story before. I have discussed the Aher legend in another context in an earlier post here.