Converting spam users to customers

Promoting a new website selling a product online can be expensive and time consuming. If you register at other websites, there is always a possibility that your id will get banned.(an experiment conducted over the last few weeks, showed that more than 50% of ids get banned even for non spam postings).
This is where having a network of your own blogs / websites helps.
If you have a WordPress blog, there are usually a lot of spammers posting comments and also registering as users. While these spammers take up a lot of database/ Web hosting space, they can also be used to build a double optin mailing list for free.
There are many free WordPress plugins which allow you to send customised mails to all users of your blog. After you clearly indicate your charges to them, most of them will not bother to post at your blog again.