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(1) to mark the limits of an area using a line, fence, rope etc. (mark something off with something):
Ex: The crime scene was marked off with official police tape.
(2) to mark something with a word, letter, or symbol in order to show that you have dealt with it
Ex: He marked off the names of all the suitable candidates.
(Retrived from: http://www.macmillandictionary.com/dict ... h/mark-off )
Everybody is getting marked off, one by one = Todo mundo está sendo assinalado ("canetado"/excluído), um por um.