I have locked my door on late students. In a highschool it
is quite different because they have to find someone to give
them a pass, or be marked with a cut. You only have to lock
the door once, and everyone gets the message.
BUT you have to lock the door at the very beginning of the
semester/year/session. Locking the door now when the class
is almost over is fruitless.
I would, however, at this point take the student aside and
say that her lateness to class, which was duly noted by you,
will be reflected in her grade, as per the syllabus.