It's starting to get a bit silly with endless unwanted posts on our forums, so..
I used to suffer the same problem when l owned a large international hobby forum and resolved the issue overnight by charging a £1 joining fee for all new members (use PayPal). This is sufficient to put off low paid 'slaves' from joining just to post rubbish but is nothing for new members if you explain why its done.
Further more, if anyone does pay and then post dirt, ban the user AND his IP address and he won't hurry back. It really does work and is a small price for new members to pay to have a clean site. The £s add up and pay for the websites, forum costs etc.
Any thoughts?