It's a good thought, but at the moment I think we have spam under control. We block a lot of IPs from potential spam areas, we have quite strict registration process, where the member needs to answer a question about table tennis to get through (which stops most spammers), and we have an active team of moderators that kills off spam almost immediately.
The method that you describe is inbuilt in the software, but risks potential new (genuine) members from posting as they can't work out what to do, so I don't think we need it as yet.