.

The problem with guest post flood control is that the only thing you can rely on is the remote IP address. Cookies are pretty much useless in fighting spam since they are just ignored by whatever script or bot is doing the posting. However, if you're just trying to prevent "real users" from posting too often, a simple fix would be to set a cookie with the time of the posting and then to check that cookie on the next post and make sure at least X seconds has passed.

Captchas are unfortunately out of the question due to them being completely unusable by, for example, blind people.