|Come for the quick hacks, stay for the epiphanies.
It's choosing between two (or more) evils. Either you allow everyone to vote as often as possible (with possible abuse), or you restrict it in someway, and have some negative side effects, ranging from forcing people to register, to blocking potential legit votes (and the fact that you can work around it if you really want to).
I take it, the OP has experienced people abusing the voting script, otherwise the whole blocking question probably never would come up.
I, personally, find forcing users to register far more user unfriendly (hostile) than the blocking of some legit votes. It all depends on the amount of visits, I presume. If you only get a handful of visitors a day, blocking based on IP probably won't hurt. If you get a lot of visitors, it might work counter productive, I agree. And yes, small sites might get big, but a lot of them will stay small forever.
In this specific case, the OP seems to have the voting procedures all done, and adding the procedures for users to log in (because I presume you think that is a better way, but I have no clue of knowing that for sure, since you don't specify a better alternative) probably takes a lot more time than implementing a simple IP-blocking test. And I personally would not do that, because of my already explained annoyance with registration on (simple) websites.
I fully agree that IP blocking isn't nice (and personally, I would just allow everyone to vote as often as possible and would manually look through the logs to see if I could detect certain abusive votes, or even write a script to do the work for me and report every Monday, first thing in the morning :), but disregarding IP blocking as useful straight when you see the words isn't helping much either. It all depends on specific cases. Most sites won't get away with it, but small sites could use it quite well, IMHO.