It doesn't matter.  Institutional investors have direct in-house ECN access, and do not use retail brokers; in many cases, they are a registered retail broker.  CIBC World Markets, for instance, is the trading arm of CIBC.  They have an inventory of millions of shares on hundreds of equities at any given moment, and will not lock up your shares just because you said so.  Trading institutions sometimes run block trades and derivatives thru clearing houses that act like a broker, or distributor, but are not available to retail traders.  Institutions like CIBC or GS can borrow shares from each other to facilitate legal shorting.  Naked shorting is also very common.  It's illegal in theory, but regulators usually look the other way.  The point is, you can't stop shorting by making a phone call to your broker.