To prevent abuse, I would recommend tiered rewards, where higher-ranked rewards cannot be accomplished via "spammable" means i.e. voting X times. For example, I think Stack Overflow's Fanatic badge to be quite effective, as it requires the user to visit the site for 100 consecutive days. Also, having a tiered system would help motivate users, as lower tiered badges would have higher level equivalents e.g. (Visit for 30 days > Enthusiast (Silver Badge), Visit for 100 days > Fanatic (Gold Badge)