By default users cannot register unless you enable them to. This is a Joomla setting.

To make the default Joomla forms more secure, and safe from spammers, you can add a captcha to your Joomla site. This tutorial shows you how to do just that.

When writing a Joomla module or component, you can access the database with the JDatabase option.

Should you have a module installed that locks you out of your back office (for instance the IP block module), you can disable this in the Joomla database.