The answer to the original question is that yes you can go into debt with the broker. But you would have to see some spike that you were on the losing end of, and very close to a margin call when it happens.

If you live in the darkest corners of Africa where the sun never shines will they be able to get that debt out of you?

Maybe they will send Green Peace after you

__________________
Regard your good name as the richest jewel you can possibly be possessed of - for credit is like fire; when once you have kindled it you may easily preserve it, but if you once extinguish it, you will find it an arduous task to rekindle it again. The way to gain a good reputation is to endeavor to be what you desire to appear.
"Socrates (469 BC - 399 BC) "