Today I have had to do some administration job on a Wordpress MU powered website, and I found arround a good solution in order to stop the spam on Wordpress MU.

Here is what you have to do. Just edit your .htaccess file and add the following lines to it:

Code:
# BEGIN ANTISPAMBLOG REGISTRATION
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .wp-signup\.php*
RewriteCond %{HTTP_REFERER} !.*haabaa.net.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) http://url-you-want.com/ [R=301,L]
# END ANTISPAMBLOG REGISTRATION
From the following code just edit this line:
Code:
RewriteCond %{HTTP_REFERER} !.*haabaa.net.* [OR]
replace haabaa.net with your own domain where you have installed Wordpress MU.
and also this line:
Code:
RewriteRule (.*) http://url-you-want.com/ [R=301,L]
Replace that URL with the one where you want to redirect the spammers.

I hope that this has made sense and it help you.