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)?
There is a prohibition against mentioning the name of a foreign deity. (Shulchan Aruch, Yoreh Deah 147).
However, according to the sources cited by the Gra there (147:3), the prohibition only applies if the name was given for the purpose of idolatry. If the name is an ordinary name which has no inherent implication of divinity and which was not given for the purpose of deification, then there is no problem with using it.
In a very interesting teshuva, R' Esriel Hildesheimer details two additional limitations on the prohibition: