Dilemma over spam - what do you think?

I've been approached by a wine company in Australia to help them sell wine online. Over the past four years they've built up a reasonable business by cold calling, distributing flyers, offering free wine tasting etc.

Recently, they bought an email list of about 100,000 business owners. They've sent eight offers to this list, offering 60% off top European wines. The emails direct people to a landing page with the offer and a buy now button.

As a result of these eight emails, they have sold almost $20,000 worth of wine and gained more than 100 new customers. That's the good news. They have also attracted numerous spam complaints, ranging from a polite 'please take me off your list' to a couple of formal warnings from the authority that monitors spam in Australia. Those warnings were pretty light, along the lines 'this person has complained, please ensure you don't email them again.'

So the dilemma is... do I encourage them to keep doing more of the same? Given they have spent less than $1000 in total on acquiring the list and sending the emails, it looks like a pretty good result.

They have their physical address and phone number on the bottom of every email, an unsubscribe link and always remove those who complain. So the list is gradually getting "cleaner".

Some other alternatives are to use Adwords, Facebook advertising, tele-marketing or direct mail - all of which are more expensive and probably slower to get results.

What do you think? Are they spammers and should be told to stop immediately? Or is there a case for continuing to send out these emails as long as they unsubscribe anyone who asks?

Or maybe a middle ground? What would you do?