Throttling frustrates power users who move quickly responding to dozens of mentions, comments and posts about me or my projects every day, but if it meant that spammers were throttled by IP, say on a time-increasing curve with each post, woot woot, and such, same could maybe be done for signup.

rule per ip address:
first comment/signup immediate
5 second timer
second comment/signup
10 second timer
third comment/signup
15 second timer
etc perhaps.
all the way up to a 45 second delay
then if you wait 60 seconds after last comment/signup submitted, then reset the clock.