Most people who take unlimited-downside positions (like selling option contracts or short selling) on a broker will be limited to losing however much cash on hand they have in liquid markets. Their broker will issue a margin call (ask for further capital to be deposited to cover any further losses), and if no further deposit is made, will automatically liquidate their position if necessary. So you're unlikely to see your net worth wiped out, you'll simply lose the amount you put in to the account to trade with.
I wouldn't count on that. The market might move so fast that the broker can't liquidate the account before the balance goes negative. In that case the broker has an incentive to try to extract the amount from the customer (if they can), instead of eating the loss on the negative balance.
