Well, first - WHY were they marked by spamcop? From what you told us in the header, your customer is SCUM (i.e. a spammer, breaking the law). In this case he has to stop that. He may think he is a valid marketer, but most likely he is not.
WHat is the difference? Well, where did the emails come from? I gues he purchased them - or collected them. So, sending them email is illegal ;) DOes he follow protocol? As in: Send with his own domain as sender, check for bounces, have a sign out link and for starters make sure people ASKED VALIDLY FOR INFORMATION FIRST?
I get a lot of marketing emails daily, and I never complained because I signed up and I can get out "fast" with one click if I have to.
Unless you handle all that, every other ISP is going to do the same - thrw you out as soon as they realize waht you do.