in reply to
Re^4: Is using 'Cookies' impractical for 'Contact Us' forms?
in thread Is using 'Cookies' impractical for 'Contact Us' forms?
How do you do 'IP-based throttling'?
You record the time of the last request from each IP address and when a new request is made you check against this record, if the last request from the IP was made within some period of your choosing then you refuse to process it.
Of course this may disadvantage users who might find themselves behind a proxy which is shared among a large number of other potential users.