1) Allow the 3 posts and trap any in excess of that. My guess is that the vast majority of "straight" folks don't post much on their first few days. They take time to get acquainted and get settled in.
2) Add a time element to the test. Legitimate posters browse, read and respond. Multiple posts in a short period of time are a good indication of ill intent.
3) Display a message telling the prospective member what to expect: "IHub makes a concerted effort to keep its boards free of spammers and other riff-raff. One way we do this is by monitoring the posting habits of new members so we can exclude offensive people from our site as quickly as possible. We doubt that you'll notice it and we know our members appreciate it."
4) Mastermind's idea has merit, though I might not grant it "genius" status.