I don't remember which mishna it is, however there is a mishna that lists certain ways a person loses their portion in the world to come. However in the case of Elisha ben Avuha, he thought he lost his portion in the world to come because a "bat kol" told him so. In the end the Gemara explains that he was eligible to repent. However, how does that fit in with the mishna that lists certain ways a person CAN lose their portion in the world to come? I'm sure Elisha ben Avuha knew which things affect whether you are/not eligible
Firstly, if one hears something directly from a heavenly source, one can reasonably believe that it's an exception.
But more to the point, Elisha had reason to believe that he qualified anyways.
The Mishna in Shanhedrin Daf 90a states that one of the categories of people who lose their portion in the world to is one who reads literature categorized as Sifrei Chitzonim.
ר''ע אומר אף הקורא בספרים החיצונים
The Gemara (Daf 100b) defines ספרים החיצונים as ספרי מינים.
In Chagiga Daf 15B we learn "אמרו עליו על אחר בשעה שהיה עומד מבית המדרש הרבה ספרי מינין נושרין מחיקו" Elisha - before he went off [Rashi] - already was so engrossed in Sifrei Minim that he used to bring them with to the Beth HaMidrash.
So he was already disqualified according to the Mishna in Sanhedrin.
The Bas Kol was about not accepting his repentance not about his losing Olam Haba. As @Yishai pointed out, as long as a person is alive and he repents he goes off that list.
Teshuva is a special benefit, which is sort of undeserved. It is a gift. The Baal Shem Tov explained that when a Bas Kol calls out every day to do Teshuva, we hear that in our hearts in the form of a sudden pang of guilt, or introspection. It is not from within but it can be cashed in.
There is a separate list of those from whom Teshuva is withheld. Most famously is the one who says that he'll first sin and then repent.
A careful look at Rashi in Va'era died that Hashem didn't actually take away Paroah's Bechira, but rather his ability to repent. Since the sudden, implanted introspective moment is a gift from above it is withheld from certain individuals. This is what happened to Acher.
If you enjoy non-literal, rational, explanations we can say that it was this lack of any moment of regret that the Gemara is referring to as his Bas Kol. But even a straightforward understanding is that in contrast to everyone's Teshuva broadcast, he got a non-Teshuva broadcast.