Here is a possible solution to preventing spam users... there is a script called Bad Behaviour which is a PHP-based solution for blocking link spam and the robots which deliver it.

It is highly effective in preventing these robots from ever even getting to your form... I am not aware of anybody that has implemented this in Moodle yet, but it is in use with programs like Wordpress and phpList, and for some people it is blocking hundreds and hundreds of spam robots a day... they never make it to the form, so they can't signup!!

Here's the link:
http://www.bad-behavior.ioerror.us/