iHeart, at least on Android, will download (using precious cell bandwidth) video ads and play them, during which time it disables the "back" button. You're forced to let the ad play. You can still switch to another app while this is going on, but IIRC you can't turn off the audio part of the ad by doing so. My "solution" when listening to iHeart is to keep the Android settings app running with the iHeart app selected, ready to "Force stop" it.