to be eligible for margin trading you had to sign a margin contract with your brokerage. They have to stay within that contract. What they have leverage over is your margin credit limit. They can manipulate that in various ways. One method I've seen is to suddenly remove certain stocks from the marginable list, if, for example, they feel the brokerage has too much exposure overall to a specific volatile and high risk stock. Let's say, for example, there's talk of XYZ company potentially going bankrupt and it just dropped 60% in the last few weeks, they might take it off their marginable list, even if the share price would still make it marginable. Scottrade used to publish their list of non-marginable stocks and it could change on a daily basis.
I've been leveraged close to my limit since I started investing. Early on, it resulted in huge losses on margin calls whenever the market took a dip. Investing in penny stocks like mCig has actually allowed me to get a better handle on things since they are non-marginable and I get full value applied against margin loans/limits when I sell them. It works a lot better when 20% of your portfolio is marginable and 80% is not than when 100% of your stocks are marginable. ;-)
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