This has already been happening Bob and is a primary reason for the present stock market state as margin clerks have been in control. The de-leveraging has been going on for many weeks and for example some very large hedge funds who used Lehman as a primary broke having their funds completely frozen.

I doubt brokerage firms would gernerally restrict margin since they are not responsible for any liability, the Client is against the assets they hold. Of course if the Client is a bad risk they may refuse margin. To many large changes here would be a radical alteration of the operation of markets but certainly they could restrict sectors or individual Clients but normally they would raise margin standards if they were concerned or felt there was too much risk exposure.