Stopping anonymous login

[Roland Whitehead]

Some friendly hacker has a script which trawls for pages without  
comments and adds one filled with links to unsuitable sites. We also  
have the same person try creating new user accounts but clearly that's  
too laborious for them so they've only done this four times.

I have removed anonymous ability to create and edit pages. I now wish  
to stop people from registering, leaving that up to the administrators.

Short of hacking Login.jsp, how can I stop people from setting up new  
accounts? I know that this has been asked before (I've found it in the  
main wiki and in the list archives) but I haven't yet found an answer  
other than "its self-regulating so we haven't done anything like  
this"...

TIA

Roland