One may wonder why Amazon has various rating systems built in (one is customer reviews, another is "did you find this review helpful?", and finally, there's one for rating sellers), whereas blogger.com and Wikipedia do not. Even eBay has a rudimentary one.
You might also wonder book authors and electronics manufacturers don't spam the amazon rating system somehow. Even Wikipedia gets spammed. In the case of eBay, this is fairly clear: to rate, you have to pay. You could sell your friends stuff and they would up your rating, but you would still have to pay the eBay fee. Similarly for Amazon sellers: without a sale, you don't get to judge the seller.
Basically, the only way to be safe from spam is to force the rater to surrender cash. A possible alternative is to get his credit card details (although any one person may have more than one credit card), and a third alternative is to keep track of IP addresses (with internet cafes and dynamic IPs, this is a weak one, though). Email addresses, obviously, forget it.