Thanks for your reply, Robby.
I downloaded the plugin and perhaps it will be useful for something.
As the authorization spam uses a) Cyrillic and b) different characters each time, I don't think you can block this spam with the plugin.
http://img155.imageshack.us/i/adiumicqspam.jpg/
I tried blocking .ru-addresses with the plugin as they appear in these buddies' info panels, but that doesn't work either.
Also a bug (?) in the program: When you block or ignore such a request, an error message appears saying "Could not add buddy for unknown reason" which is kind of obvious because it was blocked before.