What is the source for not using the proper name of an avodah zara? How far does this halacha extend? For instance if thousands of years ago people worshipped an avodah zara named "XYZ" but no longer do so, may I use that proper name when referring to it? What about if only a select few people nowadays actually worship something as avodah zara, may I not use its proper name (let's say 7 people decide to worship, for example Bill Cosby, would I no longer be able to use his proper name even though the amount of people that worship him is miniscule)?