Michael Stephenson earlier alerted me to suspicious activity with a significant number of new, very similar, users registering in a short period of time.
Sure enough, it looks like an attempt by spammers to gain access to a potentially valuable online resource.
I've now tidied up the offending accounts. However, to prevent it happening again, I've switch the user registration process to require administrator approval. Consequently, any new users will likely have to wait a little while before their account is activated.
Hopefully, this won't cause any great inconvenience and will minimise our exposure to the undesirable.