We've been wondering that for several weeks, now. Another suggestion was to require an email confirmation link to sign up.
Not sure if any of those would do the trick. May be more trouble (to do the suggested solutions) than we know.
An email confirmation might be useful, and maybe a captcha to filter robots, but banning an IP address isn’t likely to be very effective. IP addresses are relatively easy to change if you’re minded to do so (turn off your cable modem or DSL modem for a day and your ISP will probably assign you a new IP when you turn it back on). Not only that, but if you run your posts through a TOR network the IP the freerepublic servers see will always be different and will bear no relationship to the poster’s actual IP address; to defeat that bugbear, you’d have to ban all of the IP addresses for the exit nodes on the various TOR networks - the list can be had (I think) but it puts a lot of work on the server and isn’t necessarily foolproof.