just to explain, there are programs these 'spammers' have written that requests a webpage from your site, and 'fakes' the referring site to be some string they specify.
If they are hitting a 404 error quickly, the spam plugins won't help much. You might be able to put something into your htaccess/et al that would immediately 404 anything off your old blog, just so apache doesn't even have to try and search for the directory...
CG-Referrer is another good referrer-spam-blocker, and with every release I update the blacklists (named bots, keywords in the URLs, and specific IP addresses). It catches about 99% in general, 95% when the spammers start playing around with homonyms or switch to new domains or fake up words... I've tried to engineer the blacklist 'phrases' to try to not have false-positive hits.
I also now look for
http:// injections in the current requested URL, and stop on that as well -- there may be certain plugins this is incompatible with, but frankly those plugins should change their methodology. (It's used for redirects to other sites in some cases, but could be done encoded when needed, or as a hidden POST variable, rather than in the clear...)
I'll have a new alpha release of the CG-PowerPack up tomorrow hopefully, now that I've fixed one or two of my plugins to function better with WP1.5...