yes, agreed, there NEEDS to be something done about stoppong the daily spam delete that is now part of my admin duties, :-(

' daily spam delete ' is the biggest mistake one can make !
you're destroying the *evidence* that describes the spam --
so that then - no further research can be done to analyze the spam content to
derive any detection algorithms that could auto- detect and block future spam ;-(

I use a walled garden and divert the registration to me with link to check. Here is an example of the link it sends
http://www.stopforumspam.com/api?ip=27.153.209.11 

To date, I have not had one spamer get in.

If they are spammers then I report them to stopforumspam 

paypal allows a special payment type of $0.01
so that once a new user wants to register -
they must enter their paypal account details and
actually pay $0.01 via paypal to you. 
the amount is of no consequence
- however all the personal they have to have entered
to obtain a credit card, paypal account matters and
can be user to trace back to them *ppersonally*
spammers will *not take that risk route,
nor the extra work all this involves
and so.......
bottom-line = if they hafta pay  -> no spammers ! lolz !

Payment can't be for registration. I too thought of that, however due to the scale of the project, registration must stay free.

Walled Garden is already enabled, however if we try to sent an email per every registration, the mail box will be full in no time.

Why couldn't you check the ip against the stopforum database, if it is a know spammer ip then registration dies if not a known spammer then allow registration to occur. 

@RJ: payment for $0.01 only - but collects reliable data of registering user => safety @no spammers !

I think PayPal charges 2.9% + $.30 per transaction

IconMatrix: You should know because you use PayPal for Registration Fees and that blocks spammers dead in their tracks.

I think this could be added to a registration routine that was automatic, check the SFS db for bad ip's, continue the registration process if not on the list else {notify them they are spamers and where you found them or just send them to stopforumspam.com}
http://www.stopforumspam.com/usage