Adrian,
There's a few things that can help with what you're looking to do. I'd highly recommend looking at the following post:
using social network registration only, and disabling Joomla registration
. I just responded to that one with almost the same question you had. It should work for you to disable Joomla registrations, but if not, let us know in that thread.
One final note that I'd recommend investigating, if you want to keep Joomla registrations enabled, is using a Spam Protection plugin. There are a lot in the JED, listed below:
extensions.joomla.org/extensions/access-...rity/spam-protection
We actually use SpamBotCheck on this site, and it works very well. Not every spammer is denied, but we have about 5000 attempted registrations per day, and only a few ever get through.
I hope that helps,
Alex