Opening a spam post doesn't cause more spam. Clicking on a link within a spam post though could alert the spammer that the post was being read and that could cause more spam.

The main reason the spam posts are made is that google ranks search results by the number of links to a site that it finds around the internet. If the spam posts are left alone, when google crawls the site it will find those links and move the spam sites up in the search rankings. If we didn't delete them we would be playing right into the hands of the spammers.

Greg