While purging the mailing list would decrease the number of spam complaints I wouldn't send out messages to past customers with the explicit message (and only message) of opting out because... suprise they WOULD opt-out. Instead email your list on a regular basis which in-turn would allow people plenty of opportunities to opt-out.

The Gap, at least they are not pitching Viagra or Cialis. There email shows a lack of saavy about the Internet it speaks more about who is running the company as being out-of-step with the exact market they are trying to attract.