i have installed ravens anti-spam (ps raven is a fucking nutter – you should read his blog!!) – this works by creating a random string unique to each installation which has to be filled in on registration (done automatically if javascript is present) as the bots generally dont have javascript engines at their disposes they fall over at this point and cant register.
fingers crossed it will last for longer than a few weeks and i wont have to delete 200 bots a month as i am at present.