You can still throttle the spammers by limiting the frequency of posts based on IP address
Yeah, if you want lots of both type I and type II errors. In IPv4, a throttle will cause collateral damage for people behind the same carrier-grade NAT, such as students living on campus in the same dorm complex or residents of a country that doesn't have quite as many allocated IP addresses as the United States.
Hah, tell me about it! Sometime around 2004 or so, Slashdot managed to ban just about every broadband user in Swansea, the second-largest city in Wales, population 200k-500k (depending on how wide an area you count). Why? Because back then the major broadband supplier, NTL (now Virgin Media) ran everyone through a set of caching proxy servers that didn't preserve the original user's IP address. Obviously somebody in Swansea had been trolling/attacking Slashdot, and as a result the latter just put all the Swansea proxy server IPs onto their banned list, cutting off an entire British city.
Hilariously, when I emailed Slashdot support to explain the issue, I got a reply that seemed to think that "city of half-a-million people" actually meant "a few dozen people in an office", because the guy seemed to not understand the issue at all, and told me that until I, me personally, managed to find the troll and get him to stop hitting their servers, then the IP range would remain banned. And this from a major tech site. You had to laugh, or else you'd cry...
Anyway, I forwarded both mine and his emails to NTL (cc'd to Slashdot with a note to say that perhaps they might like to work together on the problem) and eventually, well over six months down the line, it got sorted by someone, somehow, I don't know the details.
In the meantime I got round the issue myself by manually sticking the address for another city's NTL proxy server into my internet connection setup (Bristol, if I remember correctly; not sure why I didn't use Cardiff, possibly too congested).