It does not seem possible to prevent students from seeing feedback and grades for an assignment, short of hiding the assignment altogether.
Sometimes it is useful to hide grades and feedback from students, at least for a limited period, because:
(1) the marks may be provisional (e.g., subject to moderation);
(2) it may be considered better to release the marks to all students at the same time.
Currently the only way of achieving this seems to be to hide the assignment altogether. But this prevents students from resubmitting where appropriate. It also means that teachers can't mark work that has been submitted early, before the deadline.
Could we have a setting for each assignment that prevents marks and feedback being given to the student (through the assignment or through the gradebook)?