How to Disable Registration completely?

Hi. At present we do not want anyone to be registering for the site. So I have disabled registration in my settings, as well as putting in a captcha (Sweet Captcha plugin) in order to deter the spammers.

However spammers are still being able to register and they are adding up quicker than Wanguard can get rid. Of course I would of liked to have used Anti Splog, but it seems to require Multisite.

Do you have any suggestions on how I can stop these bots/users from being able to register even when I have disabled it? I get a new one about every 10 minutes landing in my email.

Regards,
Ashley