This goes back to the early 1980's. I used to hang out until the wee hours with some folks at a local 24 hour donut shop. The owner had rigged-up a stereo inside a locked box, in the back room. It was set to a local "Elevator Music" station, and everyone (even the employees) hated it... but there was nothing anyone could do... until I came along.
I used an "FM Converter" (remember those? You could listen to FM thru an AM-only car radio) which I modified for direct audio output from the detector. This fed into a basic amplifier system, and into a home made FM transmitter. I would be sitting in a booth with my friends, and could not only change the station to almost anything we wanted, but also adjust the volume, bass, and treble. They were suitably impressed.
So, each time I'd come in with my device, they'd say, "Here comes OVERRIDE!" (their new nickname for me). I would then proceed to knock out the elevator music, and tune in to the local rock station. Everyone loved it.
I also made a smaller version that I could connect to a Walkman cassette player, and play my music over any other FM radio in range. All it took was a few milliwatts of power. Fun times.
Now I am a Broadcast Engineer... and I get to play with real transmitters and control systems, etc.