We've had some rather unpleasant advertising messages posted on the site over the last few days. These were created by an automated attack, in which someone's computer found our site and create some fake user accounts to post messages. I've cleared out these fake users and the offending messages.
I've added some extra software to the site that requires new users to read a code from a picture and type it in before their account is activated. This is easy for humans, but very difficult to get a computer to do, so it will protect against automated attacks but still allow legitimate visitors to sign up and contribute to the Forum.
I've erred on the side of caution while removing the fake user accounts, and I've almost certainly removed some legitimate ones in the process. If you find you can't log in, you'll need to create a new account.