What is the earliest source in the Torah where a leader/sage made a ban against something and people ignored what he said?
The Gemara Megillah (5b) says that Mordechai originally instituted the full status of Yom Tov to Purim including a prohibition on melacha, but the nation didn't accept this.
מלאכה לא קבילו עלייהו דמעיקרא כתיב שמחה ומשתה ויום טוב ולבסוף כתיב לעשות אותם ימי משתה ושמחה ואילו יום טוב לא כתיב
That depends on your parameters for the question.
Technically, Chava touching the Etz HaDa'as after Adam prohibited her from doing so is understood to be the first non-Divine command issued by an authority transgressed by others. The gemara explains that Adam was mistaken in his thought process behind the ban and commanded Chavah to neither eat nor touch it, but failed to clarify that the prohibition against touching was his extension and not an actual Divine command, in violation of Bal Tosif. This is why she does not distinguish between the two parts when asked by the nachash, and why his midrashic counter argument of pushing her against the tree persuaded her. See Sanhedrin 29b; Rav Nebenzahl also has an article that discusses this among other points.
Barring that, there were a number of nevi'im during the First Temple that were repeatedly ignored. Though most of the time they were telling people to keep already existing commandments, some made it clear that we shouldn't have been bringing korbanos, though it's a stretch to consider that an actual ban.
Otherwise, @mevaqesh is quoting the earliest source I can think of re: a gezeirah the tzibbur actively ignored, rather than one that was preemptively dropped before implementation by the rabbis.