Cant tell what version youre running, keaven, since youve deleted it from your source, however there is a nifty little plugin available from here:
http://redalt.com/
The plugin name is armor and perhaps owen, or someone else, can provide the full link, I couldnt find it.
This will help combat that sort of stuff.
The other thing you can do if youre not running 2.* (OR even if you are running 2.*) is just restrict access to wp-register.php IF theyre not coming from your domain.
There is a publically available script thats responsible for what you experienced. the path, wp-register.php is hardcoded into it. so a simple
RewriteCond %{HTTP_REFERER} !^http://([^.]+.)?yoursite.com/.*$ [NC]
RewriteCond %{REQUEST_URI} ".*wp-register.php$"
RewriteRule .* - [F]
in your .htaccess will also thwart that crap.
hope that helps :)
PS: It is also a good idea to keep bots off wp-register.php as well via robots.txt