No, if you have a Margin account they cannot touch your shares as long as they are fully paid securities and you do not have a margin debit.
They can only loan out shares that you have pledged as collateral when you borrow against the equity for purchasing stock on margin loan.
That is called re-hypothication. And brokerage firms can only loan out up to a max of 140% of your margin debit in securities. The rest of the fully paid securities are segregated and cannot be touched for the firms own obligations or loan department.
ie: if you have 1,000,000 USD in stock, and you borrow 10,000 dollars to purchase stock. The firm as allowed by SEC regulation could take up to 14,000 dollars worth of stock as collateral and use it for it's stock loan department(for short sellers). with the rest 986,000 being segregated and not loaned out
Stipulated in SEC 15c3-3
Brokerage firms make money on the interest you pay for borrowing to buy stock and the also make money on the interest people pay to borrow your stock
Let me add the risk of shorting a stock like HLF. Lets say you have stock in HLF and you pledged it for collateral, now a short seller has borrowed your shares to sell short. Now this individual is short and has an outstanding short HLF position.
You decide to pay your margin debit or sell other stock to cover it and you now have no margin debit (ie all your securities are fully paid for) Guess what, now your brokerage firm has to return the stock back to you (ie HLF) and the short seller can be forced to cover his short position, and have to pay whatever the prevailing market price is for HLF is at the moment.
The other issue with short selling is harder to locate stock will have high borrow rates, it can even go as high as 50-60% APR if it is classified as a hard to borrow stock. And it can happen overnight.
ie your paying 7% to borrow HLF for short then two days later BOOM 48% APR.
And if a short squeeze situation occurs, in addition to being forced to cover you can easily go broke from one day to the next. So what Ackman is doing playing with fire, especially with a stock like HLF.
I forgot to add, if your short stock and it goes ex-dividend. Guess who pays the dividend, you do it comes out of your pocket.
So to make money shorting, you have to be right but at the right time and be very well capitalized.