It's worth noting why this is such a selling point. I remember when Sirius Xm was ad-free. NPR is always ad-free and I often let the dial rest there. Spotify, Slacker and Pandora all offer ad-free services for a price. All of them have ads if you don't pay. (Some even if you do) Yahoo's LaunchCast Debuted as an ad-free service in 2003... but eventually introduced ads. In 1998 Lucent Technologies and CD Radio talking a big game about their ad-free satellite radio service. It never happened. In short, ad-free programming is rare and ad-free services are an endangered species.
But they noted that they were inspired by a change at 92.3 WBMP-FM "Amp" a New York City area station.
You may remember in the 4th season of WRKP, which aired in 1982, the fictional station went ad-free and became ranked as the 6th station in Cincinnati. But real ad-free radio programming is harder to pull off. But back in 1988 when Channel 63 in Los Angeles flipped calls from KTIE-TV to KBEH-TV they introduced ad-free weekends. In 1980 WCBN-FM went ad-free when they received the news that John Lennon has been murdered. They did it again in 1985 as a stunt "Commercial-Free for a Free South Africa." But that was not the first either. When 105.5 WWWM-FM flipped formats to Free Form FM as 'M-105" in March of 1975 they did an ad-free weekend. That is one of the earliest references I can find.
But If you go back to the origins of radio Lee deForrest and David Sarnoff both envisioned a radio band free from advertisements. He mourned the presence of ads on WEAF and even then in the early 1920s was reminiscing about the unstructured broadcasts of a decade earlier because they were commercial-free.