If you want less of something, tax it.

That truism is often used as an argument against a tax on profits, or health benefits, or employment, but in the case of the rents extracted from the economy by the financial services industry here's hoping it proves more of a promise than a threat.

The International Monetary Fund has put forward two new taxes on banks to pay the costs of future rescues, one of which is a fairly conventional "Financial Stability Contribution," with an initial flat levy on all banks, to be refined later into something with more precise institutional and systemic risk adjustments.

More interestingly, the IMF is also proposing a "Financial Activities Tax," (FAT) a tax on bank pay and profits which, if correctly designed, could serve as a tax on rents -- the unwarranted spoils -- of the financial sector.

In economics the concept of "rents", essentially the extra money a given individual or industry is able to extract from its clients above what it would if there were perfect competition, is central. If there is only one cable television provider in your neighborhood you will know what I am talking about.

In financial services, the evidence is that rents are huge, in part because of impaired competition and in part because increasingly complex financial services allow banks to sell clients products that they don't understand, may not need and will almost always be over-charged for. Bank employees in turn charge hefty rents to their bosses, boards and shareholders, each of whom, as you journey up the organizational chart, understand less about the complex services, and like clients, are then less able to defend their own interests.