I usually don't let the artist behind the song dictate whether I download it, so you either have to be pretty consistently terrible, music-wise, or be someone so morally reprehensible that I cannot in good conscience give you my coins.
I can only think of two artists offhand that fall under either umbrella: Ed Sheeran and Chris Brown.
Michael Jackson also comes to mind. Can't support him and his ways. I'd say Ke$ha, Demi Lovato and Eminem but they do have a (very) few bops that I use from time to time and might decide to download one day.