All brokers are mandated to provide the legal terms of the account and margin accounts include language concerning a) the risks and b) what options the broker has available should your account fall below their required margin (which might be higher than the regulatory minimum).

Brokerage accounts, mortgages, credit cards are not click-thru agreements. There is real money involved and real risk to both parties. Not reading the terms before signing and using is an unacceptable excuse for future loss or hardship.