Have you found the solution for hiding Activity Report from student?
I have the same issue.
I tried Users --> Permissions --> Define Roles --> Students --> Edit
and set Prevent for both 'See user activity reports' and 'View course activity report'. But it didn't work at all.
Your help is appreciated.
I figured it out.
Please go to Administration --> Users --> Permissions --> Define Roles --> Students --> Edit
and set Prevent for 'View own gradesand' under Course section. It will remove Activity reports tab.
If you Prevent 'viewowngrades the students won't be able to see their grades!!
Also, if you prevent 'View course activity report' and 'View own grade Report' at site level you will define it for the entire site - which you might not want later or for some courses.
The easiest way to prevent students from seeing their Activity report is to set the "Show activity reports" setting in the course setting screen to NO.
NOTE: Students cannot see their full activity report or anythig at site level, only once they are in a course and only for that course.
I hope this helps