Hi, I have a feature suggestion that I think will be very useful without requiring as much work as some suggestions here.
Even with the Google Captcha people can easily sign up it seems.
I think adding a maths question option would make it harder or maybe questions and answers that I could have add relevant to the site.
I know a determined spammer could get around that too but I find just adding a preference box worked well for a time and even still got rid of most attempts. Most hackers target sites because they have a list that comes with the software of vulnerable sites and wont go to the lengths of custom making a bot themselves just for the purpose of getting into an individual site.