I was – still am really – a moderator on another forum and we used to get absolutely masses of spam – mainly internet drugs, 'cheap' finance (if that is not a contradiction in terms) and porn. The forum used to get so flooded with the stuff you couldn't get to the genuine threads.
We eventually stopped it by ensuring that new members registrations went to a moderator for authorisation first – if we didn't like the look of them they were binned! – and secondly we put in a question which required someone to answer. Most spam get done by bots (or whatever they are called, automated wotsits anyway) and they don't do answering questions, even simply ones.
That would help here, not that I have seen any spam here, probably don't look at the right time, but if it happens as it obviously does then action must be taken to stop it.
Chris