"If it can be abused on the net - it will be abused on the net!"
I saw it coming when Google announced "Google Tags" to boost your Places listing for a paltry $25! It was an instant page one landslide. All of a sudden, anyone with $25 a month to spare could change the nature of page one.

You didn't need a place of business. You didn't even need an address - as long as mom would let you use hers (can't use a college dorm). Overnight page one results of certain businesses were littered with red balloons and serious web sites were pushed to page two or three.

Google caught the error in judgment and got rid of the "Tags" ads. No matter what reason they give for "retiring" them, the masses were abusing the system for chump change - and Google was smart enough to shut it down.
The Search Engine Intellectuals rarely factor-in what idiots will do to their great ideas - when given ample spare time, and no supervision!

Good or bad Google Places Reviews can be very suspect!

On one side you have bogus good reviews - on the other, legal vandalism! Idiots always abuse freedom, then we all pay for their actions.
Google has always been honorable about addressing abuses of their system, and I am sure they will take some actions eventually. Just requiring a login is not enough - anyone can create ten Google accounts in ten fake names in a single day.
Once you have been flamed, about the only thing you can do is delete the Places Account and start over. Just hope your detractor is not dedicated to your downfall and forgets about you. There is nothing keeping people from stalking you wherever you go.

Whatever can happen in the real world can happen on the net. Only difference is - there's no restraining orders online.