The US Federal Reserve requires a margin account to maintain a minimum of USD 2,000 to be allowed to borrow from a broker. Therefore, once a margin account balance falls below USD 2,000 the account loses its margin privileges and all positions have to be fully paid. The account will effectively function as a Cash account. If the margin account balance falls below USD 2,000 and the account was using a margin loan to hold a position greater than the account's Net Liquidation Value, IBKR will act to liquidate positions to close the margin loan. In addition, if the account is holding any amount of a negative cash balance no new opening orders will be accepted. You will first need to close all negative currency balances before attempting to place a new order. Once a margin account's balance goes back above USD 2,000 (or the non-USD equivalent) it will regain its margin privileges.
For more information, please visit the SEC's web page for Understanding Margin and scroll down to "Before you Trade- Minimum Margin".