Go to the artist screen and press the heart button twice, so it becomes a circle with a line through it. This bans the artist and excludes them from radio (you can do this with albums and tracks as well - see info on favorites)
edit: if you don’t like the artist, you probably don’t have them in your library So I don’t think you can ban them until you add something, which is counter-intuitive.
So I don’t think you can ban them until you add something, which is counter-intuitive.