FYI, any random posts or thin content posts that feature a link are looking to increase the number of inbound links to their site.
This is an old SEO technique designed to boost their ranking in Google. It's out of date and can do more harm than good these days but it still happens.
Usually it is automated (including the actual post content) which is why you see posts that make no sense. Software is used to post millions of forum posts automatically.
One of the best ways to fight it is to increase the difficulty of your registration process, ie more difficult captchas etc.
Hope this helps.