Never mind. I found Account section in the Condition block:

That's what I need!

How can I close all open positions when "Free margin"/"Margin level" of account is below some given level?

You're right. Just connected it with "Once per bar" and it works as expected.

P.S. Anyway I suppose that "Once per tick" should be the default behaviour, because "Sell" was in "onTick" tab.

Now it doesn't work again... ((
Just dropped the only "SELL" operation and it does nothing. Please fix it.
In attachment you can find the generated file.
0_1496388085831_TestSell.mq5