An ip address is not unique to a persons computer. For example, when college students try to use the campus internet access...everyone will be blocked. I think this is a realistic problem for you, because it seems like the type of site where word of mouth could drive a spurt of traffic in a short period of time.
ISPs often cycle ip addresses between users. Your ip right now, could be used by your neighbor in a minute.
If you're still bent on using an ip, maybe consider only throttling uses per ip within a limited time period. Maybe use the user agent string + ip, which might help out a few users.
But, considering someone might send automated submissions and hog your website service, at least blocking by ip would force them to use a proxy server, which would probably slow them down a lot.