Was it possible that someone from West Africa crossed the Atlantic before the late 15th century? Sure it was, just like it was theoretically possible that Chinese ships crossed the Pacific. It is however extremely unlikely, first of all because there was no reason to do so, second of all because the rich islands off the coast of Africa that were not visible from shore remained inexplicably uninhabited (which would make no sense had there been a major ocean-faring capacity in the general region), and third because we have absolutely zero evidence (besides oral history about the departure of a one-way trip) to suggest this was the case. There is no unexplained presence of African ecological or biological markers in American societies from that early a date, and no evidence of influence or borrowing aside from artifacts that extreme Afrocentrists have elected to analyze based on the conclusion they already believe. The historical consensus is such a firm "no" that most serious academic writers on precolonial Africa (Ehret, Thornton, et al) don't really even address the prospect beyond noting that the story exists.

One can never prove with certainty that something did not happen, especially when there is no evidence that authoritatively confirms or denies it. But the preponderance of evidence points to a negative conclusion--even though I would absolutely love for it to be true, because of the prospect for tracing elements of African society and culture that may precede the great shocks of the Atlantic Age.