I have noticed these posts too, usually so obvious because they are new members, posting at 2am, and give a long story that often doesn't make sense, along with the link. I am not afraid of offending any of them here because I would be surprised if any come back to see the replies. Replies aren't the point, getting people to click the link is the point.
I found out all about this when I was setting up my website, (no link, sorry!) and when you want to be at the top of Google search results, you need, among other things, other websites to link to yours. When Google searches the web, it finds the sites with the most traffic from the most sources. So, because adding links to a bunch of other websites (it's easy to do this in a forum) gets more traffic to your site.
My suggestion would be to not allow new members to post links, otherwise the posts will be deleted. You could ask that current members could be the arbitrators and identify what we consider spam. Otherwise, unless anyone is really upset by them, we could just ignore.