The last mark on that wall was only half finished. This means she stopped part way through making her last mark. Possible reasons:
a. She just decided randomly for no reason not to follow the geass any more.
b. She was still trying to finish the mark, but someone physically pulled her away from the wall for some reason.
c. The geass wore off and she didn't finish the mark that day.
Which of these seems most likely to you?
a is impossible from what we've seen. I can't think of a reason why b would happen unless someone got upset that she was defacing school property, so I suppose that might be POSSIBLE. I'd say c is easily possible, though.
The geass has a time limit.