There are two things you can do, the first is to certificate them but this is not obviously to everyone’s advantage but the alternative solution is simple. All you do is to phone your broker (or do it yourself) and put an order in saying that you wish to place your shares for sale at, for arguments sake, double today’s price. As they are 'on order' they cannot be lent out by your broker and in turn you are reducing the amount of 'free shares' out there that can be used for shorting purposes. And don't forget to move your limit order up when the price starts to recover, then, that way your shares can't be shorted
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