OK, so I actually found an issue with the Captcha we use on here. For some reason, it wasn't displaying. This was allowing the bots / spammers who could get past the first line of defense to be able to register. KeyCaptcha was installed, and I haven't touched any of the settings in a while now. The option to use this was missing from the admin section :confused:
I've uninstalled, and re-installed it, and it's working again.