Stopping robot registrations
One of the steps we did in Civi was to set our Available Countries. This does not allow user registrations unless they are from within one of the countries on the list. Most of the spammers are operating in specific countries, so if you are not needing the whole world to have access to your forms, then you can do this to block out a large % of these guys.
Admin&gt;Localization&gt;Languages,currency,locations then scroll down to Available Countries and add the one(s) that matter to the approved list
Joomla might have something similar as well that you can add there.