Can have loose safegaurd via capture of X_FORWARDED_FOR header and register this IP with vote then server side do sliding 30 minute window check for multiple votes by same user on same topic (should be good enough to stop initial spammers!!!)
recording IP addresses? You'll have comrade DW on your back! Is anything possible on the client side, e.g. a cookie that tracks which discussions you have voted on? Not foolproof by any means...