Mod_security so far has been quite effective at beating off the referrer spammers since I installed a few rules. It seems to have gotten even better since using blacklist_to_modsec to generate a bunch of rules based on MT Blacklist.
According to my stats, my server went from serving up at least 50MB of bandwidth/month to spammers (11 000 + requests/month) over the last two months (and that was just 403, 404 and 500 errors) to just 550kB (1500+ requests) so far this month (my default mod_security action is to send a 412 Precondition Failed error). They're still there and banging away, but instead of having to serve up the entire page requested, all they get is a 350 byte error page. That means a heck of a lot less bandwidth that I have to waste on spammers. There are still a few sneaking through, but it's not nearly as bad as it was before.
To paraphrase Flo, Kiss my bits spammers.